From my experience...
People claim MK is broken in singles, but get this-the only double team which is really worthwhile is double MK, and virtually every team that places well has at least one MK in it. Check the results. MK is not only obscenely good in singles, but in teams... Well, Tornado is basically instant and powerful pressure (that plus a strong move can easily wreck shields, plus your teammate can stop you from picking up any heat from it). MK has a perfect recovery, plus can save his teammates from being gimped pretty effectively. MK can carry, support, and does shockingly well in 2v1s-better than any other char. He really belongs up there.
Snake is really good for various reasons... Best stock tank in the game, won't get gimped as much, incredible kill moves, C4 gimmicks, incredible damage output... Oh, and grenades are amazing.
DDD... That ****ing Dthrow man. That ****ing dthrow. DDD is an incredible assist, a great stock tank, and can effectively shut down one of the opposing team for a good while with grab gimmicks. His strengths really shine in teams, plus his bad matchups (Falco, ICs, Pika) all suffer greatly in teams, at least in what makes them good against him.
G&W has some ridiculous gimmicks in doubles (uairing MK's uthrow anyone?), survives fairly well, and... well, come on, the guy probably has the most viable kill power in the game. He's AMAZING in teams.
Wario is good for a lot of the same reasons G&W is good, his Dthrow is like DDD's, only slightly worse, and he has a lot of **** going for him, including REALLY good damage output.
There's (probably) your top 5... A few top tiers who shouldn't be making the list are Falco, Diddy, and ICs, mostly because they kinda suck in teams.