Falco, Pit, Snake, then Toon Link. Reason being is that Falco has always been my favorite since before Melee (always will be), but Pit is nice when I just feel like messing with my opponents. Snake, however, is pretty awesome, being that he got me through I think Classic mode on the hardest difficulty. Toon Link is great for the Boss Battles mode on Extreme (imo).
In short, it's Falco for sentimental reasons.