The answer to your question really depends on how you play and what kinds of games you enjoy.
Do you play with friends? Or do you prefer to play alone? Do you like racing, RPG, strategy, platformer?
I'll provide my picks for all:
Alone
- Super Mario Galaxy (platformer)
- Excite Truck (racing, but not simulated racing -- and yes there is a multiplayer option, but the single player kicks ass)
- Zelda (RPG)
- Paper Mario (platformer / RPG)
- Metroid (1st person shooter / strategy)
- Zak & Wiki (strategy / puzzle)
- Trauma Center 2nd Opinion (unsure how to categorize this one)
- Fire Emblem (strategy RPG)
With Friends
- Mario Strikers (sports? heh, not really but fun)
- Wii Play (if you need a 2nd controller, this is the best option)
- Rayman (the 1st one)
- Warioware
So quite a bit more than four, but pick a variety from these to keep you happy for a good while.