Easily to me, mario sunshine. That game took all of the great success from mario 64, and completly revisioned it while remaining to the original forumla. I played it through almost 7 games and is one of my favorite games to date. It was underrated because it didn't sell as good as the others.
The most overrated, imo, has to be galaxy. All they did was take mario 64, cut, copy, and paste it to a wii disk and added space. It was pretty dull to me mostly because all the other marios (besides sunshine) had the exact same storyline. Thats not to say it was a bad game, just that its the most overrated in my book.
Your most underrated/overrated mario game???