Resident Evil 4 - Amazing game. Should be much fun even if you've already played it (though if you have, you already know whether you want it or not). This is a must buy, for sure.
Okami - I've heard good and bad things about the Wii version. Supposedly it's worse than the PS2 version, which is too bad. On the other hand, it is still a great game; similar to Zelda but much better in my opinion (I played it on PS2).
Williams Pinball - Lots of fun from this bargain bin game. It's not your typical bargain game (i.e., it's actually good).
One that I've heard are good but I am not sure myself:
MLB Power Pros - A fun baseball game and much deeper than Wii Sports baseball. Has licensed teams, players, and stadiums, despite the cartoonish appearance. I rented this one just before I sold my Wii.
Super Paper Mario - Don't know. If you love Mario and you love RPGs I'm sure you'll enjoy it. If not, you probably won't.
Pro Evolution Soccer 2008 - Apparently it's very different on the Wii than on the other consoles, and for once it's different in a good way. The control scheme is totally unique. I read a few reviews and the general consensus seems to be that it's quite challenging to get the hang of it, but it's a really good game once you do.
No More Heroes - Never played it, haven't even seen gameplay footage, but people seem to like it (even Yahtzee liked it, and that's saying something). One of the few "grown up" Wii games available.
Bully and The Godfather have gotten pretty good reviews too, but of course they're remakes of games that came out on earlier systems. I think those are both sandbox-type games.
Oh, and you should definitely rent Boom Blox before you buy it. It's fun, but I don't think it's worth $50.
EDIT: OP, Resident Evil 4 is a third-person shooter where you walk around and shoot guys. The rail shooter you're thinking of is Resident Evil: Umbrella Chronicles (or RE:UC).