This is too hard.
It's probably Venice, Barkhang Monastery or St. Francis Folly.
Venice because it's one of the most beautiful levels designs ever and nostalgia.
Barkhang Monastery because it's really fun IF you don't shoot a monk. Idk, I just love this level so much, there's nothing about it that I don't like (again, if you don't shoot a monk.
)
St. Francis Folly is amazing. That feeling of isolation, very few enemies and you're surrounded by cool puzzle rooms named after ancient Gods. It's fantastic both in TR1 and TR4.
Ok, there are more levels I LOVE, but at least I tried.