Currently I don't think anyone is truly OP. I don't think every matchup is even though. I feel like some characters DO have an advantage over others. Roughly 50% of my online 1v1 matches have been played as Bowser Jr, 40% or so have been Ness and that last bit has been spread out over about 6 other characters so I won't pretend that I know how every character stacks up against every other character, but I'll give you my input on some of the ones I've seen mentioned in this thread so far.
Bowser - Not OP. Don't get me wrong, he's a really strong fella. He seems a good bit faster than in Brawl and if he's reading you better than you're reading him it will be a short match, but so many of his moves are punishable. Even the poor guy's side smash has a windup period where you can smack him out of it pretty easily and then if he misses it you can smack him while he's laying like a big sexy naughty turtle on the ground. His breath is good for gimping, his Up B has taken priority over some of my aerials, and I've even fallen victim to getting spiked by his dair, but I wouldn't consider him OP at all. He still takes good reads to play effectively.
DK - DK is surprisingly good imo. He has a combination of range, priority, and power that makes him hard to deal with in competent hands. His bair beat out all of my aerials when I faced him as Bj. Every trade was in his favor since he hits so hard. He may be lacking in projectiles but the battle I had with him may be the most frustrating matchup I've had as Bj yet. He might be easier for a character with more projectiles or more speed.
Rosalina - Rosalina is annoying. Her moves cover good area around her, they hit harder than I would expect, and they don't give a very big window afterward to punish. I thought she would rely more on Luma than she does, but Luma just feels like an afterthought. Rosalina doesn't need Luma to be a threat. She's still not OP, mind you, but she does feel like she requires more reading than a lot of characters in order to punish effectively.
Captain Falcon - I'm not sure how I feel about Falcon yet. He's quick and somewhat strong, but I haven't seen the knee in action on this game yet. In melee that was the reason I played Captain Shinynips at all. That knee was amazing. In Brawl the sweetspot felt different, I didn't like it as much. In this game I still haven't seen how strong it is so I'm holding back full judgement until I see it.
Ganondorf - I kinda put him somewhere between DK and Bowser in the sense that he has a lot of power, some good range and priority with some of his moves, but he does require that you know when to use the right move. I fought one who abused his Sparta kick and the fact that I suck at teching out of the chokeslam. He also managed to side B through my fair which made me very unhappy. Against someone faster than Bj Ganondorf is probably easier to deal with, but this one player that I faced just dominated me like 8/2 or something.
Greninja - I don't think Greninja is OP. I said it. I have a friend who fell in love with this character and plays him A LOT. While Greninja is fast and tricky, all of his moves can be dealt with reliably once you find the right method. Like fighting a Zelda boss or something. You just have to stay alert.
Little Mac - I admit that when I first got this game and played Little Mac a bit I was almost disgusted at how easy he was to dominate with. So much speed. So much power. And that KO punch... When I first faced one online I was a little worried how it would turn out, but he's actually not that hard to deal with (at least for Bj). The same strategy seems to work for me almost every time. I stand with my back to an edge and throw out some projectiles to encourage Mac to close the gap, he does. He uses 1 of about 3 moves, 90% of the time it's the dash attack. I shield and grab, throw him off the edge, position myself horizontally aligned with him in the air and throw out a fair which he side Bs into. I jump out and give him another one to put a little more distance between him and the stage. He plummets to his death at about 40%. Repeat. Sure, sometimes it takes a few attacks before I catch them with the shield grab, and sometimes they'll throw in a counter instead of a side B, and the KO punch has to either be respected or baited out at 0% damage, but eventually this strategy always prevails. Always. Little Mac is probably the single best character for dominating beginners, but because he's so easy to edgeguard he'll never be top tier.
ZSS - This is another character that I usually only see players use about 5 of her moves in a match. That's not the character's fault really, but so many players I play online get into this habit of rolling back, shooting that stun thing, and then trying to dash attack or (in rarer cases) grab after it that they become easy to deal with just cause you know you'll be getting the same pattern of openings every time. The only ZSS I've ever faced that gave me trouble (and she gave me a lot of it) was some hyper aggressive player who rushed me down harder than anyone else I've ever played. She destroyed my first stock in embarrassing fashion. Unfortunately for her, "full speed ahead" seemed to be her only setting. I started dropping exploding clown cars on her and she ate every one without playing more defensively at all. I think if she would have added just a little bit of defense to her strategy she would have won every match against me, but knowing that all she was going to do all match was charge at me made things too easy.
For me, my hardest matchups have been DK, Ganondorf, and Yoshi, though not necessarily in that order.