Obviously this topic has a lot of subjectivity, but it's great fun anyway.
I'd think of classics as . . . if you could only have XX number of games (25, 50, 100, whatever) in your MAME cabinet, what would they be?
Then, there's the list of wow that's a fun game to play, or I've never played that before, I just discovered it and it was cool style of games.
Anyway, there's so many great games . . . that's what makes designing a control panel so difficult.
I can't believe Robotron didn't make the first three lists on this post!