1. Advance wars days of ruin- Great turn based strategy, multi card play and wi-fi and technically single card play. Make your own maps, so easy too! Addicting storyline and 26 levels for campaign make this game assure you at least 30 hours of gameplay, your first month! I've spent almost 40 hours so far. Cons? Like many strategy games, after playing more then 40 hours, gets kind of boring. Too easy to go on defense, espically on wi-fi battles or friends.
2. Fifa 07- It's soccer people, not much more. Sure there's only multi card play and no wi-fi, but in my opinon fifa 08 and 09 were too real. Too hard to foul people =) Fifa 07 is the easiest fifa to make crazy goals. I've spent a good 20 hours on this game, all soccer!
3. Mario Kart- People, it's mario kart, not much more to say. You got single card play, multi card play, and wi-fi. 32 tracks instead of standard 16, battle modes can actually have computer controlled "bots" so if you're ever bored and have no human friends to play with, you could do that instead of always racing. Cons? Omg the in free for all in battle modes, the bots only go for you, I'm not even joking. Look at the mini map and you'll see the bots release their red shells only when they come close to you. Wi-fi can only have 4 players :P, kind of weak when local you can have 8. Too many snakers, or I suppose drifters, in wi-fi.
4. Custom Robo Arena- If you like fighting games, you'll love this. So many customizations, that's why it's "Custom" Robo Arena. Not much more to say about game. Check it out, you got a LOOOOOOOOOOOOONG storyline, + wi-fi, multi card play, and single card play.
5. Metroid Prime Hunters- only fps so far that has wi-fi, I wonder how cod5 will be like. Storyline is pretty good but not enough save points. Only 4 players, local and wi-fi. Bots can be put for multi card play, or even by yourself if you're bored. Cons: Wow you can't even disable health packs, kind of annoying when you're shooting someone and they run away to get health...