I agree about Tiesto producing other tracks better than ST, but it was one of the first trance tracks I ever mixed cleanly, and for that reason alone it always reminds me of the feeling of elation. Adagio for Strings is a wonderful piece of music, but I prefer Samuel Barbers original version to be honest!
Firewall - Sincere is another belter, but it's not really a dancey track so I tend not to use it in sets too much. It's the same for Adagio really, which is why I prefer the original, classical piece.