Well really it depends on how much time you have on your hands. Pokemon obviously needs a fair amount of commitment as with Advance Wars.
Games like Pokemon and Animal Crossing and Metroid Prime if you like FPS are alot better online or at least having friends with the games.
So I don't no if you have wi-fi or not, but some games just aren't all that great when you're not playing online.
Unless of course you're more into single player games, in which you could get some RPG sort of games like Final Fantasy (also Pokemon) and CastleVania.
Tetris DS seems very good in all aspects to me. You can have endless fun in single player, multiplayer and wi-fi. Plus there are all the different modes of it. You can also just switch it on and play for 5 minutes whenever you have time.
And you can never be too good at Tetris (unless you're me of course )
ive played animal crossing wild world and i thought it was crap! oh and a brilliant game to get is Elite Beat Agents! on the touch screen numbers appear with a circle round them and when the circle gets tight round the number you tap it. you go along to the rythm with different songs such as Sum41 - Makes No Difference, Chicago - You're the Inspiration, Madonna - Material Girl, etc.:smilewinkgrin: