From when I played it, I can tell you that I didn't really enjoy Monster Hunter Tri... I was probably expecting too much, but it's kind of an unworthy sequel to the previous MHs.
As far as Mario Kart Wii goes, I think it's actually the best title in the series. I know you SNES minions will stamp down on me, but yeah... MKW is more fun to me... by a lot... and it's got an amazing online mode.
As far as Smash Bros goes, I was enjoying Melee a bit more than Brawl, seeing how it had more unintended techniques you could abuse. Brawl definately is a monster of a game, and a ridiculously fun multiplayer game, though.
The Mario Galaxy's are great, while I thought the second one couldn't quite keep up with the first. Amazing games, though, with lots of fresh ideas.
Definately Metroid Other M, as well as the Trilogy. Opinions regarding Other M are rather split, so I wouldn't give a 100% guarantee that you like it... if you're a fan of the old titles like Super Metroid, though, you should find a game you like.
So much for that, now for the underground titles noone ever mentions...
Disaster: Day Of Crisis is a rather different point-and-shoot game with a lot of interesting game design catches. It's got a certain humoristic aspect to it, and while it seems to take itself quite seriously, you shouldn't
It's got lots of unlockables, too, so if you're a greedy mofo like me, it offers quite some replay value... I played it for a good 30 hours in total.
It's the successor of SOS, a Playstation 2 game, if you played that.
Then of course, No More Heroes. It's certainly not everyone's taste, but personally I think it's one of the most amazing games released on the Wii. Be sure to check it out, but don't buy it blindly.
Then there is Sin And Punishment 2, which I have unopened on my desk, unfortunately (we're doing construction work at my house atm). If it's anywhere near the quality of the first (which is available through Virtual Console), it's a definate must-have for your list.
And while I don't get how noone could've mentioned it yet: Wario Ware: Smooth Moves. It's simply the most amazing game on the Wii to date, and there's no way I'll let you miss it. I got about 200+ hours on it.
Gotta warn you before the fact, though: It says multiplayer, but those games are essentially fugly and not worth it. Play by yourself, and you'll have fun!