techincal Fox's are a struggle but if you play like a *** it gets easier. I have to change my state of mind in order to fight fox though. It's like charming a snake. You have to be willing to stick your nose in dangerous territory all the time but be ready to move out of danger when he Fox makes a move. After you get use to that it's easier to play.
Falco's almost strictly laser problems. If you don't avoid, counter, or shield/shield-reflect his lasers extremely well he's gonna be a HUGE pain in the ***. If you have a friend with good laser game just play him on infinite time and just practice your anti-laser game until it gets easier.
Idk...I find Fox and Falco difficult but at the same time I always feel like I'm gonna beat them lol. Gimping them is so easy it's ********.
As for my least favorite to play it's gotta be Sheiks. Good sheiks are difficult to beat but you can do it if you play smart. Gay sheiks on the other hand are an entirely different animal. If a sheik is playing like a ****ing ****** there isn't alot you can do except wait. It's my least favorite for that reason.
Aside from that I've struggled at times against Peach's or Puff's but it's mainly do to myself and the fact that I get bored using Marth against characters I can't **** with consecutive hits