Zack and Wiki is a good rental. The puzzles are interesting and the expressions of everyone are funny as well.
I've found the following to be the best games that I have played:
Mario Galaxy: Obviously
WarioWare: This is a fast and furious minigame masterpiece. It is a great party game and is very addicting. Not the best for just one person because it does not take very long to pound through all the games.
No More Heroes: This is a VERY good game. I'm working on it now. It's very out there. If you enjoyed Killer 7 then you will likely enjoy this as well since they have very similar styles. Not for the faint of heart though. Very violent. The gameplay is a little repetitive, but it is so off the wall and the boss battles are quite varied that it helps compensate.
Resident Evil 4: This was one of the greatest games ever created even BEFORE it was on the Wii, and the Wii just makes a masterpiece even greater. I would choose this over Umbrella Chronicles ANY day.
Metroid Prime 3: A very fun game. If you played 1 or 2 (prime) the gameplay is very similar but tweaked to employ the Wii Controls. If you liked 1 or 2 you will probably enjoy 3 as well.
Trauma Center: Second Opinion/New Blood: This game is more obscure but in a good way. You get to use the Wiimote to perform operations on people. You won't be doing a lot of "real" operations (you will be dealing with one primary disease that mutates a lot and you will deal with the different versions.) The difference between the 2 besides the missions is that New Blood allows two surgeons to play at once and it allows you to upload your scores to Nintendo Wi-Fi. I would recommend playing Second Opinion first though, seeing as you can likely get it quite cheap as it was a launch title, and it will be hard to go back to that if you play New Blood first.
Super Monkey Ball: Banana Blitz: This has a LOT of good multiplayer and party games. Another launch title so it will be easy to get cheap as well.
Oh yeah Paper Mario is nice as well. A lot of extra stuff to do after you beat it, but if you just want to play the regular mission it may be better as an extended rental, or used.