I think for Buddhist category better to divide into 2 groups : Indian Subcontinent including SE Asia, and East Asia because of different ideas of architecture, art and sects.

Indian Subcontinent including SE Asia (12)

Ajanta Caves
Bamiyan Valley
Borobudur
Buddhist Vihara at Paharpur
Golden Temple of Dambulla
Kandy
Lumbini
Mahabodhi Temple Complex
Sanchi
Sigiriya
Takht-i-Bahi
Taxila

East Asia (12)
Dazu
Haeinsa Temple
Hiraizumi
Horyu-ji Area
Kii Mountain
Longmen
Mogao Caves
Mount Emei
Mount Wutai
Potala Palace
Bulguksa Temple
Yungang

http://www.worldheritagesite.org/categories/category14.html

Borobudur - iconic "great" Buddhist monument
Potala Palace - underepresented sect of Buddhism and it most famous landmark

Unfortunately, I do not know enough about India to make a proper assessment, they all look exceptional.

Is it possible to narrow down which Chinese (and Korean) Buddhist grotto/cave is the most exceptional? Longmen, Mogao, Yungang, Seokguram/Bulguksa? Perhaps narrowing it down to 1 or 2? Thoughts?

Indian Subcontinent including SE Asia (12)

Ajanta Caves - YES, impressive art, the fresco and rock craving are the masterpiece.
Bamiyan Valley - NO, famous for its Taliban destruction, few significant sights remain
Borobudur - YES, one of the most iconic Buddhist monument.
Buddhist Vihara at Paharpur - NO, the vihara is one of the biggest but the style can be found in many ruins from Bangladesh to India.
Golden Temple of Dambulla - NO, rock painting is very intricate, but for Sri Lankan Art better to represent by Polonnaruwa.
Kandy - NO, the Temple of Tooth is maybe one of the most important site for pilgrimage but its architecture is still not the Sri Lankan's best.
Lumbini - NO, Birthplace of the Lord Buddha, but almost nothing left.
Mahabodhi Temple Complex - MAYBE, the birthplace of Buddhism, the center of Buddhism pilgrimage, but the architecture is controversial.
Sanchi - NO, one of the masterpiece of Indian art, and the early model of stupa, but other stupas in Sri Lanka are more impressive.
Sigiriya - NO, this is fortress or palace or temple is still not sure.
Takht-i-Bahi - NO, important site but similar sites can be found.
Taxila - NO, one of the most important sites for Buddhism education in the old time, but I will prefer the tentative Nalanda.

East Asia (12)
Dazu - NO, its Taoism element is far more superior.
Haeinsa Temple - NO, the temple is beautiful but the WHS is the storage for wooden blocks.
Hiraizumi - NO, paradise gardens are beautiful but Japanese garden can be represented by more important site eg. Kyoto
Horyu-ji Area - NO, oldest wooden temple does not mean beautiful or iconic.
Kii Mountain - NO, Buddhist elements can be found in Kayasan and Yoshino, but Kyoto and Nara can represent this similar style.
Longmen - NO, one of the best Chinese art during Tang Dynasty, but Yungang can show more styles.
Mogao Caves - MAYBE, another gems of Silk Road, the paintings are one of the best in East Asia
Mount Emei - NO, biggest Buddha in the world, sacred mountains similar to Mt Wutai.
Mount Wutai - NO, another sacred mountain
Potala Palace - YES, represent Tibetan culture and very iconic
Bulguksa Temple - NO, one of the nicest temple in East Asia, its grotto is a masterpiece but very small compared to caves in China
Yungang - YES, the only grottoes that show various styles of Chinese art development, the motif and still colorful.

Its smaller because its man-made, which is unlike any other grotto I am aware of.

Buddhist architecture seems to have developed in its own way in each region / country, so I would go for a geographically wide spread:

Ajanta Caves - the question will arise if we can have both Ajanta and Ellora: yes I think we can
Borobudur - iconic sight, one of the greatest Buddhist monuments in the world
Lumbini - it's either Lumbini or Mahabodhi as representation of the holy places of Buddhism
Sanchi - oldest Buddhist sanctuary in existence
Sigiriya - most spectacular site in Sri Lanka, used by Buddhist monks
Taxila - important archaeological site, ancient place of learning

Dazu - coming together of Buddhism, Taoism, and Confucianism
Mogao Caves - " the largest, most richly endowed, and longest used treasure house of Buddhist art in the world"
Potala Palace - to represent Tibetan Buddhism
Seokguram Grotto and Bulguksa Temple - to represent the Far Eastern / Korean tradition in Buddhist art

Ajanta Caves -
Borobudur - iconic sight
Sanchi - oldest Buddhist sanctuary in existence
Mogao
Potala
Emei
Mahabodhi- Most sacred in buddhism (better than Lumbini in architecture )

I agree with Els to go for geographically wide spread ...

My selection :

Ajanta Caves (possibly the best together with Ellora)
Lumbini (birthplace of Buddha) as well as spiritual place for several Eastern cultures
Borobudur (iconic and one of the greatest WHS)
Sigiriya (Sri Lanka's best site and quite unique in Asia)
Potala Palace (iconic and one of the best WHS worldwide IMHO)
Mogao Caves (maybe)
Dazu

I also want to vote for Dazu, Sigiriya and Luang Prabang. Luang Prabang is known as Mecca of Theraveda buddhism (one of the sects, another is Mahayana; includes every other site)

Now, Lumbini does not have anything (just like Bethelem). But, both are of huge religious significance.

Yes, having decided (rather surprisingly and probably unjustifiably in my view) not to include Bethlehem (partly on the grounds that Jerusalem is enough to cover the life of Jesus? It can't be because it "does not have anything" - the current Basilica goes back to 565 and has been in use ever since), we are faced with a decision regarding 2 sites associated with the life of Buddha.
a. Mahabodi Complex at BodhGaya - I have been there and would offer my review as an indication of what is there and the history of its identification, authenticity and management - http://www.worldheritagesite.org/sites/mahabodhi.html
b. Lumbini. I haven't been there, but this short BBC video might be of interest in showing what the main Maha Devi temple looks like.
http://www.bbc.co.uk/news/world-asia-28744451

So we have 2 sites which certainly contain some original remains which "probably" relate to Buddha's period of birth and enlightenment and certainly to their use as shrines/temples soon thereafter. Both sites achieved "rediscovery" in the 19th C after many years of non use (at least by Buddhists - Mahabodi continued as a minor Hindu shrine). Both have a range of ponds and other places associated with the story/legend of Buddha. Following their rediscovery both underwent development in the 19th and 20th Cs. At Lumbini that development doesn't claim to be authentic but covers the original ruins. At Mahabodi a 19th C English archaeologist attempted to recreate a structure similar to what the original is thought to have looked like from models etc. Both have been heavily criticised for the developments which have taken place and both still have major management problems. Both are important sites of Pilgrimage and it would seem that one of them ought to justify itself both for its possible historical authenticity and for its more recent role as a major pilgrimage site?.
So - how to choose??
Form what I have read/seen and (but only in the case of Mahabodi) I would choose the Mahabodi Temple.

Seokguram Grotto

Is this world class? Unfortunately, I cannot make a holistic comparison with some of the Buddhist grottos/caves in China having never visited those particular parts of the country.

"The Grotto is a masterpiece of Korean Buddhist art, without equal in the region." AB Evaluation

I've been to both Lumbini and the Mahabodhi Temple. Both have significant Buddhist sites nearby (Tilaurakot, Sarnath) and both have a spiritual dimension (intangible). Yet this time round, I'd opt for the less 'iconic' Lumbini to represent Buddhism. As a non-Buddhist, I felt that Lumbini is more comparable to Jerusalem for Judaism or to Mecca (non-WHS) for Muslims. That's why I chose Lumbini over Mahabodhi.

While I agree with much of the selected WHS that represent Buddhism, is it not strange if the current selections are finalized, no Buddhist Temple will be included in the voting? How many churches and cathedrals do we have now (including maybe's), how many denominations? I can't help to think we are inevitably entrenched in our Western perceptions when regarding East/South East Asian heritage, no?


In addition to my previous selections I would also support:

Mount Emei (Location of the first Buddhist Temple in China, and largest Buddha statue in the world)
Seokguram Grotto/Bulguksa Temple (Masterpiece of Buddhist Art, representative example of Pure Land Buddhism)

Dear Els, I think Sigiriya should not be considered as Buddhist category. It main stucture is the Sigiriya rock which was a palace fortress. The later addition buddist monastery is very small and well hidden in lower section which can be seen when we climb down from the rock almost at the exit which I think better to be a "connection" not category, and Sigiriya should be considered as archaeological site.

As a buddhist, we not considered Lord Buddha similar to Jesus Christ, so the significant of Lumbini is very different from Bethelehem. Lumbini is one of the four places that associate with the life of Lord Buddha which are, Lumbini, Mahabodhi, Sarnath and Kushinagar. But for the religious itself, Mahabodhi is maybe the most significant as it is here where Lord Buddha enlightened and doscovered the Buddhist doctrine, so its significant is beyond the life of Lord Buddha.

For Luang Prabang and theravada buddhism which is my believe. We never consider Luang Prabang as Mecca. There is no pilgrimage there at all, also Laos is not considered a spiritual destination, it maybe for non buddhist by tourist marketing. For Buddhist pilgrimage, we go to Mahabodhi temple and other sites relate with the life of Buddha in Nepal and India. Other lessen WHS pilgiramage site for both Mahayana and Theravada is Kandy (including Adam's Peak). Also we do not consider Burodudur as holy sites as there is no sacre relic there.

My selections:

Indian Subcontinent including SE Asia:
Bamiyan Valley
Borobudur
Lumbini
Mahabodhi Temple Complex

East Asia:
Mount Emei
Potala Palace
Yungang

My selections:

Indian Subcontinent including SE Asia (12)

Ajanta Caves - Y - for its carvings
Bamiyan Valley - Maybe - not sure what's left without the Buddas, but as per the discussion somewhere else: if we take Palmyra, why not Bamiyan?
Borobudur - Y - fabulous site
Buddhist Vihara at Paharpur - N - ruined site, other more impressive
Golden Temple of Dambulla - Maybe - interesting and somewhat different site
Kandy - Y - major religious site in Sri Lanka
Lumbini - N - not many remains left
Mahabodhi Temple Complex - Y - for the mahabodhi temple itself
Sanchi - Y - major ancient Buddhist site
Sigiriya - N - as per Durian, this might better fit into another category
Takht-i-Bahi - N - major Gandharan site, but Taxila seems to have been more relevant ?
Taxila - Y - major Gandharan site

East Asia (12)
Dazu - Y - amazing carvings (as per discussion, might fit into other category too)
Haeinsa Temple - Y - seems to be major inscribed Korean temple; the wood block collection gives it an additional importance
Hiraizumi - N - prefer Kyoto
Horyu-ji Area - Y - I disagree with Durian on this - Horyu-ji was for me one of the most lovely Japanese temples I saw in the area of Nara and Kyoto
Kii Mountain - N - prefer other Japanese sites
Longmen - Y - haven't been to any of the Chinese cave sites, but the photos of the Longmen one have amazed me the most
Mogao Caves - N - see Longmen
Mount Emei - N - prefer Wutai for sacred mountains and Bamiyan for giant buddhas
Mount Wutai - Y - already supported this on the basis of CL
Potala Palace - Y - the quintessential Tibetan site
Bulguksa Temple - Maybe - Bulguksa temple is very nice; nor sure how it compares to Haeinsa (which has the woodblocks); Seokguram grotto is nice but really small
Yungang - N - see Longmen