The more obvious titles got mentioned already. (Metroid Prime Trilogy, Xenoblade, The Last Story, Super Smash Bros. Brawl, Super Mario Galaxy, Zelda, etc.)
If you can find a copy (one of those typical low-print-run Atlus releases, from the looks of it), have some friends, and don't mind putting your friendship with them at risk, there's also Dokapon Kingdom, which is more or less Mario Party-meets-JRPG with the potential for screwing over your friends turned up to Munchkin levels.
(Of course, it's not a Wii exclusive since there's also a PS2 version, but neither is Resident Evil 4, whose best version is the Wii version due to half-decent controls for once and the PC port being a total trainwreck without a heap of mods.)