While they're probably the most mainstream you'll find, my favourites include Pachelbel's Canon in D, Vivaldi's Four Seasons and most of J.S. Bach's Well-Tempered Clavier. I also like Nessun Dorma, whose composer's name escapes me at present.
I must confess, alas, that I haven't listened to anything particularly exotic. Most of my experience with classical music comes from my parents' TimeLife 50-CD set they'd bought in the mid-90s.