It's a mosque, not a temple, but I also love the Nizamuddin Dargah in old Delhi. Again, it's a very important site for Muslims and non-Muslims alike, contains a vibrant and expansive religious practice, and is absolutely gorgeous, architecturally speaking.

Ajan is being overly aggressive, but I have to agree with him that in terms of both its construction and the sheer vibrancy of its current practice, the Venkateshwara and Padmavati temples at Tirupati are absolutely astonishing. (The two temples are dedicated to form of Vishnu and his consort Padmavati, or Alamelu in Tamil.) The Padmavati temple is at the foot of the seven hills in Tirupati, and the Venkateshwara temple is sits on the top hill. It's a huge and ancient temple and because the form of Vishnu within it is known as a boon-giving deity, it's also a hugely popular site with pilgrims. According to the wikipedia entry, over 50,000 people visit the temple everyday. (http://en.wikipedia.org/wiki/Tirumala_Venkateswara_Temple) Yikes! Along with Srirangam (which was included on this list), Tirupati is one of the most important temples in South India and is one of the 108 Divya Desams, or holy shrines that were canonized by the twelve Tamil Iyengar saints. (It should be pretty clear from this post that I have a special attachment to this particular temple--it's one of my all-time favorites!)