Zelda/Metroid are amazing games, so in my opinion the 350 you would spend to get a wii, Zelda, Metroid, and Wii Sports (also extremely fun) is well worth it. After that it would depend on your taste.
Raving Rabbids and Warioware are great fun for multiplayer (and I enjoy Mario Party 8 every now and the as well.) but if you aren't interested in multiplayer you might wanna rent or pass.
On purely single player, Resident Evil 4 is worth it even if you played it on GC/PS2 as the controls make it almost an entirely new experience. Super Paper mario was also excellent, and Trauma Center is worth it if you didn't play the DS version, or if you REALLY enjoyed the DS version.
Other than that the only games I have any time with were Scarface (just So So,) Wii Play, worth it for the controller in my opinion, Red Steel (also just so so), Madden (meh), and some of the cartoon games (do not get, even if you are a fan)