Hmmm... I think the Gameboy..Advance was the worst one because it was basically a uglier Gameboy color. The only difference was they made the games smaller and added more pixel like graphics..
Hell i still play my GBA. The only games i play are 2 different castlevainias and a yoshi game, all three games kik @$$ and also Double Dragon isnt bad, anyways i think the worst would be, aww i cant remember the name but when they released they actually LOST money lolz
Ive only played.... the gameboy color, GBA, GBA SP, and ds. Gameboy color was bad cuz it didnt have a backlight like someone mentioned and the screen was small. The GBA's design was kinda dumb, randomly putting the screen in the middle. The GBA didnt really have any flaws. And the DS is just awesome. The games are actually really fun, it doesnt just have brain games, those are just the ones they market.
When I was little, we had this think it wasn't rare but it wasnt common either, it was called the G-O-D (gee-oh-dee) and it had sonic on it, and the joystick was like the PSP and it was lame...
I'm going to have to go with the PSP and the DS. Only because, after all the advances (PS3 & Wii) they left us with what appears to be a primitive device that they just threw together. In addition, the relative limitations that they uphold on these devices is ridiculous. You can't FREELY use homebrew (there are always fixes and methods to try and keep the PSP/DS locked), the graphic output on both devices is completely embarassing. After all the time spent on creating the devices, they left us with PS1 style output. And, most of all, the web browser (which at generally sounds impressive) can't handle many web pages available on the internet, and require the web masters to consider creating a PSP specific page. Same goes with the DS...
wait, dont pick on the game boy color..it was pretty revolutionary for its time, in fact it saved me from a lot of boring trips, and when i think of the topic worse hand held, I dont think it fits. Because at the time when the game boy color was in production it was consider as the best handheld syatem(well only actually) so just remember the time lines it like comparing a 67' Mustang with a 09' Ferrari Enzo.