Core:
Super Mario Galaxy
The Legend of Zelda: Twilight Princess
Metroid Prime 3: Corruption
Super Smash Bros. Brawl
Resident Evil 4: Wii Edition (In case you've already played this last gen, I'll add an extra core game)
Resident Evil: Umbrella Chronicles
Okami
No More Heroes
Casual:
Mario Kart Wii
Mario & Sonic at the Olympic Games
Boom Blox (Which is kind of a mixture of both, and extremely fun)