Here's what I find myself playing the most -
DrMDx:
Road Rash II
Streets of Rage 2
Shining Force II
Gunstar Heroes
Hu6280:
Galaga '90 (best Galaga ever!)
MAME:
The Punisher
King of Dragons
Knights of the Round
Tempest
Q-Bert
Final Fight
Cadillacs and Dinosaurs
That's all I can think of off the top of my head. There are so many good games available that I'm sure I'm leaving out some of my favorites.