Personally I hated Metroid Prime 3. I thought it was way too hard, and it's not a real Metroid game-they've moved away from trying to do Metroid-style gameplay like the first one did. Of course many people like it, but I'd recommend renting first.
Personally the only Wii game I've liked so far is Super Mario Galaxy. It's the first 3D Mario game that IMO actually deserves the Mario name. Still not up to par with Super Mario Bros. 3, World, or New, but it's actually really good, and would take longer than a day to beat.
It's also the first (only?) Wii game that actually impresses me graphically. Other than that, if I didn't know better I'd think the Wii was less powerful than the Gamecube, because so few games seem to be doing even as much as the better Gamecube games did even though it's basically 50% more powerful.
And yeah, Resident Evil 4 is awesome too, if you don't already own it for the Playstation 2 or Gamecube (I haven't played the Wii version, but assume it's very strong too).
I guess those would be my two recommendations. Fire Emblem on the Gamecube was one of my favorite strategy/RPGs, but I'm not sure how the Wii one will stack up, as reviews have been very mixed. I thought the first Gameboy Advance game was way too hard, and from many reviews I've heard the Wii one is too hard also.
You can pick up Gamecube games too, if you pick up a controller and memory card. There are a lot more great Gamecube games to date IMO (check out the Gamecube thread on here to get some ideas). Problem would be actually finding the games at this point.