The first, and best, game of the series.
On the surface, this game does not appear to be a any special. The list of playable charcters is somewhat limited, the list of multiplayer levels is really limited, the single player can be finished in under an hour, and the basic rules of gameplay are very simple. However, if you have a group of friends who are into it, you will find yourself playing for hours on end at the risk of dropping out of college and alienated loved-ones -- not that I have any personal experience with that.
The gameplay is very easy to pick up for the first time and very difficult to master completely. Each player can jump and attack, that's about it. However, each character also has a very specific range of motion and attack. So if one was so inclined, any player can take their character of choice to a very advanced mode of precision.
And, of course, what gamer wouldn't want to see a bunch of iconic Nintendo characters beating each other up?