Super Smash Bros. Brawl is my favorite fighting game because it is fast paced, chaotic, and has extremely high replayability. You can jump in to the action with friends and have a blast regardless of their skill level. You can never predict what is going to happen. There is a high variety of stages, amazing music, and a huge roster.
Unlike other fighters, there is more luck and less combos/skills. While some may view this as a negative thing, I view it as a good thing. It balances the playing field and gives an element of unpredictability. Overall, SSBB is my favorite fighting game I have ever played.