Super Mario Galaxy is tops, no doubt about it. I love the improvements made from 16-bit Mario to Mario 64, and I love the improvements made from Mario 64 to Sunshine. If galaxy doesn't absolutely floor me in its awesomeness as the other two titles did, I'd be stunned...so I expect the best.
Super Paper Mario is up there nowadays too. I've largely ignored it until recently when I heard that it has been given a bit more of an action/platformer feel versus so much of an RPG engine? Is tha true, or did I misread something? Because I liked the first Paper Mario in a sense, but some of the RPG aspects just turned me off totally as most RPGs do (turn-based battles...).
Sonic and the Secret Rings is next. Looks like after each 3D Sonic iteration, they've finally made the one that most appeals to what I loved about Sonic. As long as the gameplay is not flawed in some horrible way, this is a must-own title for me.
Wii Play is next. I can't wait to play most of the titles in this game, billiards especially. The reviews of the international versions have solidified my excitement.
Then there are the obvious choices: Super Smash Bros. Brawl, Metroid, Mario Kart, Madden 08, hopefully EA does something with the Live series, and the list could go on forever. I can't remember the last time a console had so many must-own titles (for me) in its first year or two.