Typically safe characters that don't have to commit to anything too much see: Sonic (particularly) and Diddy. Charizard's neutral is very "eh" and largely centered by NAir (which is probably his best move) and he can't approach very well. Charizard shines when it comes to punishing and edgeguarding, and is absolutely terrifying at an advantageous position. He's the character that wants to adapt to the opponent and punish their mistakes accordingly. Unlike other heavies, Charizard doesn't really excel at anything, rather his moves are multi-purpose and diverse so he doesn't fall into typical heavy problems see: he actually has no trouble landing safely. Also, rage benefits him. A lot.
Which goes back to my point, basically any character that are difficult to trap and punish are probably hellish for Charizard. Since he doesn't really have Bowser's OoS option or DK's pokes these character really don't have anything to fear against Charizard. He just doesn't have that one thing that makes you respect him, if you know what I mean. (I guess there's Flare Blitz, but that's not something you should throw out too much against a good player) If he can't get into a good position, then he's going to have a lot of problems in the match.