My Game Gear collection has increased quite a bit since I made my original response a couple posts back. I can't make any suggestions as to what should be added/removed from the top 20 list, but I can share my current favorites.
-Castle of Illusion starring Mickey Mouse
-Columns
-Devilish
-Dynamite Headdy
-Halley Wars
-Jurassic Park
-Land of Illusion starring Mickey Mouse
-Legend of Illusion starring Mickey Mouse
-The Lost World: Jurassic Park
-Mortal Kombat
-Mortal Kombat II
-Ms. Pac-Man
-NBA Jam (it's slower, but still fun)
-Slider
-Sonic Spinball
-Sonic The Hedgehog
-Sonic Chaos
-Sonic Triple Trouble
-Streets of Rage
-Streets of Rage 2
-Super Monaco GP
-Super Off Road
-Tails' Adventure
-Tom and Jerry: The Movie (Yes, the movie sucked, but I was surprised to discover that this was a pretty decent platformer, despite the short length)
-Woody Pop (well worth checking out if you like Arkanoid and such)
-World Series Baseball
-X-Men: GamesMaster's Legacy