His recovery both ways is the best of the super heavies, even without customs; though to be fair, when your only competition is garbage in either one department or the other, that's not all that hard to do.
But he's definitely the most versatile of the super heavies, too; he can play spacing and can play the close-up game and does both pretty well. The reason I place Charizard above DK and Bowser is because they have a harder time recovering than Charizard does, even in spite of being technically stronger; they only really have one recovery move each, and DK's is hot garbage if your below the stage, while Bowser's is at least decent, but suffers from being faster and going farther horizontally than vertically. Flare Blitz does wonders for Charizard's recovery, since it basically gives him one of the best horizontal recoveries in the game, albeit at the cost of some chip damage, while Fly is a great OOS option and good for getting back onto the stage from below the ledge.
His aerials come out fast and combo well into other moves, and having Flamethrower to shut down projectiles is a great benefit for him, since it lets him get in close without too much risk. His jab is extremely quick for his weight class, too. DK and Bowser are good, but they still have big flaws that limit their potential (basically, they both have one-note recoveries which are easy to gimp). Same with Ike, although I'll concede that Ike is definitely close behind Charizard in terms of viability.
I don't think he's at B+ tier, but I'd hazard that maybe the middle of B is a good place for him. I cannot see Charizard as being worse than
Little Mac. I mean, I love Little Mac, but there's no chance that he's better than Charizard, even with his ridiculous ground game.