Hrmr, well considering I own both a DC & my current phone is an N-Gage classic... Uhmmm I'll go with Sega Mega-CD and Sega Game Gear (8 AA's to run it?!)....
As for DC, I still love my Powerstone, Crazy Taxi and a couple of other arcade games on it... N-Gage is currently holding Civilization 2 & Settlers of Catan (if you don't know what those are, bow your head in shame and go read wikipedia.org)