I've gotten to where I don't have too much trouble with most players, but I still struggle against ROB and Diddy players, and depending on who's playing her, ZSS can give me trouble. I'm slowly starting to get better when playing against those three though. I used to have a tough time against Ness, Fox, and Captain Falcon players, but I've gotten better with them and now win more battles than I lose when playing against them.
The biggest problems I have with the 3 I mentioned, are how fast Diddy and ZSS are. Each Diddy player I've played against has a habit of faking out and roll dodging almost none stop. ZSS seems to have no cooldown after using her smash attacks, making it difficult to punish. ROB players are just flat out annoying and do nothing but spam projectiles and use their spin attack xP