So many choices! I will pick
Temple of Xian though as being my all time favorite level. This level is simply a masterpiece and encompasses all of the elements I want in a Tomb Raider level. First off, I love the level design and the whole level is oozing with atmosphere. Just all the details like the wall paintings and dragon markings on the wall really absorbs you into the level. I also love how the level focuses on traps as the main challenge. There are no human enemies and the animal enemies are there as just an annoyance. This is how I wish more levels in TRII were like. Finally, I love the complex design of the level. For example, when you find a key you don't know where it could open so you traverse the whole level and stopping at every keyhole hoping it's the right one. The whole surreal feeling of the level is the reason I love Classic TR games.
Runner-up for me though would be Palace Midas. Believe me, it's VERY close.