The balance between the characters seems to drop with every game that's made, and I find it sad. SSB is ruining their own series, I swear. Don't think they're unbalanced? I think I can easily make a list of the OP characters:
Bowser - The bowsercide is a cheap way to win, not to mention he's fast and powerful.
He's only fast when he runs, which is when he locks up his options. If he manages to approach, it's mainly because you let him.
Yoshi - Literally all you have to do to win a game is spam your dash attack, it's incredibly sad. He's incredibly agile, has great range and disables, and to top it all off can easily stack up damage.
If that's literally all you have to do, why I don't I just drop my main, pick up Yoshi, and become the next M2K? Oh right, because that's not literally all you have to do. He's strong, no doubt, but he's not THAT good.
Rosalina and Luma - I don't think I need to say anything here, just go against someone and use nothing but your B attack and you'll win.
R&L have been nerfed and without Luma, she's just glass for about 13 seconds/780 frames. Powerless, and extremely light.
Diddy Kong - Incredibly fast and damaging multi-hits, great recovery and great knockback.
Diddy has a lot of advantages. Recovery is not one of them.
Shiek - The priority of her attacks is insane, they're incredibly fast and powerful; what doesn't she have?
You could arguably be right here, but Sheik still dies early enough to be somewhat fair.
Ganondorf - He's not extremely OP, but if you are against a somewhat-competent one you'll instantly lose.
Only if you let him approach. The whole point is to stay away from Ganon before it rains for him.
Zero Suit Samus - I think we all know why.
Up B does kill early, but so can you, considering how light ZSS is. Also, you can spot-dodge her grab pretty easily and punish back. If she doesn't grab, she's wide open.
Little Mac - Fast, deals a ton of easy damage, impossible to dodge his hits, etc. Need I mention a KO punch?
Not impossible at all. His recovery is mediocre and everything he has forces him to get up close. Perfect Shield does wonders.
Falco - Spam your B attack, you become invincible and they end up dying.
Are we talking about the same game?
Lucario - He can 1-shot a Kirby with an uncharged smash when he is at 100%. Fair, right?
This is a specific MU, a character can't suddenly be OP because one character can't handle him. (If Lucario does indeed hard-counter Kirby)
Ness - Just spam PK fire to get them up to 100%, then walk up to them with your baseball bat.
How about getting out of PK Fire range if it's such a powerful tool?
Captain Falcon - Fast, quick, and powerful attacks, he's also pretty tanky.
His recovery isn't good though. Falcon can be as beefy as he wants, he's not coming back if gimped properly.
Lucina - A side-tilt deals about 18%, and her counter is insane.
Counter can be frame-trapped easily if that's your problem.
Shulk - I could make an essay as to why he's so unbalanced.
I doubt you can.