Two Weeks with the Wii
I can't say anything for or against Rayman, having not played it.
But Monkey Ball does have 50 multiplayer mini games, which are all enjoyable to varying extents, depending on who you are. The majority are 4 player, but some (~5) are two player, and some don't use the nunchuk, so if you're strapped for cash and only have the wiimote, you're not all lost. It also has a single-player game (also using just the wiimote) which has the potential to make a grown man cry with its insanely difficult later levels, and will definitely level up your fine motor control ability. But in a good way, because you know they are able to be beaten and that they've been playtested to death to be sure that there's guaranteed to be a sliver of hope, no matter how small, that you can indeed beat the level. There are even two hidden worlds of 8 levels each that can be unlocked only after beating each world with no continues.
I vote Monkey Ball. I got Zelda and Monkey Ball and they've served me well.