Definitely Cowboy Bebop & GITS: SAC.
Lain: Serial Experiments I only liked that one opening song, but that's just me.
Iria: Zeiram the Animation -- LOTS of cool instrumentals. Good driving music.
Armitage II, same thing as ^
Heroic Legend of Arislan -- elegant, classical, moving and stirring
Ranma 1/2 is good if you're looking for "cutsey" and fun
Science Ninja Team Gatchaman -- if you remember the bastardized version called "Battle of the Planets," all the *music* is the same & you can picture the anime in your head as you listen.
By the way, I have *hundreds* of anime OST's. Used to have them all available for download on Tamarasanime.com. If anybody is looking for something in particular, let me know & I'll put it up on my server for you to d/l.