That one's kind of hard... if we're basing it on how much you liked it in its time as opposed to at this moment, then I really liked quite a few of them heh
My first handheld was actually the game gear, and I really really liked that one, though the battery life admittedly sucked.
The gameboy was cool but even back then I thought the screen was just unacceptably poor (especially when compared with the Game Gear). The Gameboy Color was a lot of fun as they actually managed to provide a color palette for games that didn't originally have one, and I got to experience Pokemon for the first time.
Largely skipped Gameboy Advanced... DS was fun, PSP was great... I guess I'd have to go with the 3DS XL just for the combined libraries of it and the DS, the larger size (very comfortable to hold), and impressive battery life while having a lot of great games.







