To me, temples can be amazing for many reasons. Some are beautiful, others a seat of tranquility.

For me, the Drepung monastery just outside Lhasa is the epitomy of tranquility (for more details, see http://en.wikipedia.org/wiki/Drepung_Monastery).

For that reason, I'd say it's my favourite. Thanks for the other comments, which have given me some ideas for future travel.