The Missing Link
For me, this game is what ties the Mario of my childhood to the Mario we see in Galaxy. The game features many of the game-play mechanisms seen in games from the NES era coupled with some of the newer innovations in gaming such as 3D graphics and save points.
That said, this choice limits the games appeal somewhat because many modern-day gamers will be put off by the old school feel of the game. The most often cited issue being the lengthy amount of text that the game requires you to read. If you can get beyond that, the story is extremely interesting and the dialogue is always pretty funny. I really enjoyed it and felt like it gave a good context to the upcoming levels which makes playing much more meaningful.
The 2D to 3D gimmick in this game functions very much like the bullet-time gimmick of Max Payne. Sure it's a gimmick, but it's one that works incredibly well and adds to the game play as opposed to just being there for gimmick's sake. I also liked how it was obvious when you needed to use it because one of my main worries was that I'd be completely clueless about which mode I should be in when.
All in all, I finished this game glad to have put in the 20 or so hours it takes to play the main story all the way through. The puzzles never seemed to get too monotonous and the story kept me engaged enough to want to know how it would all end. If you've already played Galaxy and are already missing the man in red, I'd recommend this as a nice addition.