Can't think of 10 and in no particular order -
Psycho Mantis - MGS - So not particularly hard but really fun and at the time unique, he told you what games you've been playing, moved your controller across the floor and at one point has you switching controller ports to avoid mind control.
Sephiroth - FF7 - First time heading into this I was very under-equipped and he was a bastard to beat.
R-Type - been a long time since I played but it was a hard game all round - the boss I remember took me ages to beat.
Sagat - SF2 - By completing the game on the hardest difficulty from start to finish, without continuing, you were awarded with something or other (can't remember now). That bastard Sagat sent me back to the beginning about 20 times before I finally beat the SOB.