I don't know, probably a tie between PsP and Gameboy Color. I have more games for the PsP, but Pokemon Yellow is one my most played games ever.

All handhelds suck tho

Game Boy Color, because it yielded both my first game-character crush (Shantae) and the games that made me fall in love with the monster-training RPG sub-genre (Pokemon Gold/Silver/Crystal)... I hope you can ask me this question again after the release of "Pokemon Heart Gold" (which I have prepaid in full at my local GameStop) and "Pokemon Soul Silver" (which I'm holding out on for a less hectic day to prepay), because I want to know if the remakes are any good/as good/better compared to the original games....

The Atari Lynx 2, I couldn't even start a game on that thing!

I'm the friend who preferred the GBA.  The main reason I loved it was that I never had Nintendo machines as child, so I was like a kid in a candy store with portable versions of Yoshi's Island, Mario World, Mario 3, Zelda III and Super Ghouls 'n Ghosts (and Donkey Kong Country 1-3, which came free with the GB Micro I just bought).  Ignoring those games, I also enjoyed far more original games on the GBA than I have on my DS.  Each of the following games occupy slots in my top 50 games of all time:  Astro Boy, Kuru Kuru Kururin, Advance Wars, Wario Ware, Mario Kart Super Circuit.  Outside my top 50, the list of amazing games continues: Mario & Luigi Superstar Saga, Ninja Cop, Advance Wars 2, Fire Emblem, Wade Hixton's Counter Punch (dare I say I prefer it to Super Punch out without being flamed by the OP?), Denki Blocks, Final Fight One, Sonic Advance 1&2, Sega Arcade Gallery (great version of Outrun), a decent version of Shining Force (ignoring the zoomed in visual style I disliked), Mario Golf, Minish Cap (Ok so I only got an hour in before I realised I'd bought a dodgy version off Ebay and it wouldn't save)...

And probably a handful more I've forgotten.  By contrast, I've never clicked with my DS.  The games I've really enjoyed on it can be counted on one hand - Sonic Rush, NSMB, Professor Layton & The Curious Village....and Bangai-O were it not for the game-crushing slowdown.  I didn't even like Wario Ware DS, and as previously mentioned the original is one of my favourite games ever, and The Phantom Hourglass never grabbed me (though I have no doubt it's an excellent game, along with the sequel). 

So there.  One vote for the GBA, but only because there wasn't the option to vote for a Game Gear with a Master System converter and magic batteries.

it's also battery operated.

OT: Definitely my PSP. I love it to bits and owned just about every one (except lynx, although that was amazing, and Game Gear, which was massively underrated).

My PSP goes everywhere with me, music, games, books, movies, internet, remote play on PS3 and now comics. Genius.

I'm going old school with the OG. GameBoy.

The psp is my favorite.

you really should give the ds sequels a decent go if you love these games so much. imo especially spirit tracks, bowers inside story and advance wars ds. complete them then we can talk

leave these forums and never return!!!

dS FTW!

PSP for me. The port of VP2 alone gave me more fun than anything in the DS library to date. Then of course Dissidia, Crisis Core, GOW: chains of olympus and of course the PS1 library I transfer from my PS3 (FFVII, VIII, RE1,2,3, Twisted Metal, etc), can't wait for Vagrant Story :).