I have no real best. But the one that would probably be most qualified for it would be Mr. Game and Watch. Followed up by Luigi and Jigglypuff. I'm also pretty good with Wario, Ike, and Marth. I must say, though, beating people with Jigglypuff is so satisfying to do, especially when you're up against someone who's rather smug and confident in beating you.
As for worst... That would be a tie between Pit, Metaknight, and Toon Link. Mainly because I rarely choose them. I have grudges against Pit and Toon Link, Pit generally just being obnoxious to deal with and I hate most his attacks anyway. Toon Link, however, I'm not sure why I don't choose him and refuse to. I could just be a stubborn mule with no real reason for it, no telling. But as for Meta Knight, I'm no good at using him. Although I generally choose him when just messing around and usually pick the gaudy pink colour scheme for him because of how ridiculous it looks on him.
As for everyone else, I'm rather average with them. Some ranging to above average but not quite good enough to below average but not quite bad enough.