2:






+

I could see him being top tier. He has amazing attacks that can be changed based on the situation, meaning that he can have a great combo game, a great kill game, and thus a great punish game. His only major flaw is the distance of his recovery.
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His down throw, back throw, neutral air, forward air, and up-air are amazing, but that's about it. He has troubles approaching, and his recovery is the worst of the top two tiers. PK Fire does good damage, but it's very telegraphed. Neutral B is useless, down B is very situational, and his yo-yo smash attacks are very weak.
Opinions on the others:

just above

because Diddy has an easier time killing and recovering, though Mario combos slightly easier.

was VERY close to getting the downvote, but he escaped with his great recovery and power.

is a solid contender with great frame data, a projectile, and the best aerial mobility. He does suffer from a bad grab game, though.

With his nerfs in power, he can't be top tier, but he still is a good contender with his mixups and speed.