Stock standard as intended, it has to be the Gameboy Advance SP AGS101 version - Small, backlit colour screen, plays GB, GBC and GBA cartridges.
If you want to defy Nintendo though, and start adding flash carts so you get all the NES games on top of the above, then the range of additional games playable on the DS trumps the lot. I'd go with the DS Lite, for the massive battery runtime.
Original Gameboy for me - Went everywhere with me when I was growing up, and had a huge catalogue of games, and gave me access to a lot of ports from other consoles that I wouldn't have otherwise played as didn't have any other consoles at the time.
Mario land 2, Wario Land 1/2, Kirby's Pinball Land, Pokemon Blue, Zelda Link's Awakening, Desert/Jungle Strike, Race Days, Mortal Kombat 2
Neo Geo Pocket for me, superb joystick and an excellent collection of games.