Depends on what you like, I'm just going to list a few games that are on my do not sell back list.
Excite Truck: Yes, Excite Truck. It's a fairly fun game that just lacks in content. You will play a lot of it for the first few days, put it down for a month or so, and then continue to pick it up again and again because it is really a fun game despite its lack of content. It's good for a short session of gaming and it something that's worth keeping around.
Rayman Raving Rabbids 1/2: Either of these are great, though it's up to you which one you want to get. Some people like the first one better, others like the second one. If you regularly play with friends, this is a fun game for non-gamers and gamers alike as the humor is brilliant.
Super Paper Mario: The difficulty isn't there but the writing is absolutely brilliant.
Resident Evil 4: Wii Edition: Who would have thought you could make one of the greatest games ever even better? RE4 is an already amazing game that gets even better because of the controls.
Metroid Prime 3: Corruption: I'm not a huge FPS fan and I didn't like MP1 or MP2 but I loved MP3. They nailed the controls (though play the advanced controls, the default ones are rubbish) and it's just a *really* good game except for a single pointless fetch quest. After playing this you'll wonder why they even bother making FPS games that still use analog controls.
Zack & Wiki: I can't think of a good reason why you hadn't already picked up this game other than you couldn't find it. Seriously, it's a must have.
Warioware: Smooth Moves: By yourself this game is crap but with the 12 player multiplayer it's one of the most fun times you'll ever have.