I've only gave this thread a quick scan because I'm in a hurry but I'll give my two cents real quick and get into detail later.
I agree with Ditchy and Vox wholeheartedly. I also agree with what Mr.M said about SFA. It
is a good game. I have a copy of it myself (and all the other SF's) and I can say that it really is enjoyable for StarFox fans, and even more so for StarWolf fans. Panther is down right awesome!
As for my pick of underated game, I have to go with Elite Beat Agents. It has huge crittical acclaim but didan't do as well as Nintendo hoped. It sold modestly enough for an upcomming sequal, though. If you enjoy rytham games (Parappa, DanceDance Revolution) then you will LOVE this game. It rocks on so many levels, the EBA agents are to cool for school, and the game has to be one of the funniest to come along in ages. (a washed up baseball star rising back into stardom by saveing kids from a "GIANT FIRE-BREATHING GOLEM!" will make you split a gut in laughter.
)