There might be some confusion on m part about these games' relative popularity, but here's a list of some of my favorite games for the system that may or may not be "hidden":

Astro Boy: Omega Factor
Baseball Advance
Book Worm
Double Dragon Advance
Muppet Pinball Mayhem
Ninja Five-0
The Pinball of the Dead
Rebelstar: Tactical Command
Rhythm Tengoku (Japan import)
River City Ransom EX
Riviera: The Promised Land
Scrabble (UK import)
Sigma Star Saga
TMNT
Yggdra Union