Lee
Is it a bird? Is it a plane? No it's Supermod!
Gameboy
-v-
Atari Lynx
-v-
Sega Game Gear
-v-
Game Boy Advance
-v-
PSP
-v-
Nintendo DS
-v-
Atari Lynx
-v-
Sega Game Gear
-v-
Game Boy Advance
-v-
PSP
-v-
Nintendo DS
(Also would be NeoGeo pocket, but that didn't get a UK release, and the Nokia N-gage, but I am not sure of the releases of that)

GameBoy = 118.69Million
DS = 96.22 Million +
GBA = 81.44Million
PSP = 50million+
Gamegear = 11 Million
Lynx = Under 500K
(All figures include all incarnations, for example GBA would also include the SP, GB would include the color, DS would include lite and DSi and PSP would include the 3000.
All hail the kings of handheld gaming, Nintendo. many have tried to break onto the market and many have been destroyed. Sony putting up the best fight with their PSP but recently their game library in the UK has been lacking (incidentally they have a much better US/JP game library).
So which do I pick? 2008's bigegst selling console in the DS? The breakthrough in handheld with the GBa or my trusty old friend the GameBoy? I shall go with the Game Boy, it's simple graphics brought hours and hours of fun, ranging from Tetris, to Zelda, to Mario Land without the pile of crap that the DS has. Nintendo, I applaud you.
Let the debate commence!