The WonderSwan comes in three versions: the classic, the Color, and the Crystal. Color and Crystal are the same system but Crystal has a better screen and less battery life though all of them run on just one AA.
Classic: 30-40 Hrs
Color: 20 Hrs
Blue Wing Blitz
Chocobo no Fushigi na Dungeon
A port of the playstation game in nice monochrome 2D, just as good but some prefer this for portability and such.
Final Fantasy 1/2/4
Perfect ports/remakes of the SNES/NES games, lots of the sprite work is redone and improved.
Guilty Gear Petit 1/2
Judgement Silversword -Rebirth Edition-
Shoot 'em Up
A fast-paced score attack game with heavy focus on aggressive play through eliminating enemies quickly. Your shield cancels bullets and destroys enemies, yielding multipliers, but recharges slowly, requiring a bit of tactical use. Available as a port on the Eschatos 360 release alongside its sequel Cardinal Sins. Somewhat of a rarity in cartridge form as it received a limited reprint, being made by an independent developer on the WonderWitch development kit, though it's easily emulable.
Klonoa: Midnight Museum
Makaimura for WonderSwan
Makai Toushi SaGa
Excellent remake of Final Fantasy Legends 1 for the GameBoy, this version jacks up the power to SNES like levels and is totally worth the $5 bucks or so that your average Wonderswan game costs.
Namco Super Wars
One Piece: Chopper no Daibouken
The only handheld port of the game, and it's a damn good one too.