Customs off, Sheik is probably the best character, I think most of her players accept that as well.
I personally think Sonic is still top 3, people say that Sonic isn't as good as other top 3 candidates because "he'd be everywhere/winning everything otherwise" but frankly I don't understand why he isn't. He's just amazing at everything, and that's on top of the "you can't hit me" thing. Some slight landing issues and not being great directly up close (though he's not bad at it either) are the only weaknesses I can really see for Sonic. I don't know why there's been a bit of a drop-off in Sonic lately, perhaps I'm missing something, but I agree that it's part of an out of sight, out of mind thing since many just... don't want to deal with that character. Yeah. Like I'm not 100% convinced Sonic is top 3 but if I was asked to list a top 3 right now, he'd be in there.
As for the final spot... Rosalina's customless MUs probably aren't strong enough to place her in top 3, her players think she goes even with a lot of random mid tiers as well as losing to some of the other top tiers. If everyone agreed that her MU spread was as good as some people are saying then I'd think she was top 3 but I don't think her MU spread is as good as that nor is that the general consensus. In customs, I think she gets significantly better. Luma Warp and Shooting Star Bit are both really silly customs that make her already top tier zoning even better and generally seem to patch up her iffy MUs.
Luigi has a lot of problem MUs and I cannot believe he is top 3. BUT, that isn't really stopping his tournament results. I think it has a lot to do with his worst MUs mostly being with low high/high mid tiers like Mega Man, Pac-Man and Greninja, but whatever the reason, we aren't seeing him being held back by his MUs in practice even though he should be in theory. If his results continue at their current pace, top 3 will be reasonable in future.
I think Pikachu, Fox and Diddy are all possibly top 3.
Brawler seems to rely hugely on 60% kills and HK "jank", I don't know that much about him but from what I do know he's unreliable and inconsistent at high level, which isn't good. Top 10 probably, top 5 maybe, top 3 unlikely.
I am warming to the idea of a top 10 Ness, who I used to think was a quite overrated. I still have trouble believing he's top 5, I think there are at least 5 characters better than him. Ness is the anti-meta top tier, he kills you for shielding and if you mess up even slightly trying to edgeguard PKT2 you will die, but I don't think he's categorically better than many of the other top tiers. He competes with characters who have better mobility, better camping and better recoveries (it's risky to edgeguard Ness but still very possible, unlike with Sheik or ZSS who are extremely difficult to edgeguard). He is good though. And no matter which way you slice it he's better at killing than a lot of the other top tiers, which may end up being more important than his mobility and recovery weaknesses in the long run.
This brings me to ZSS. I have been so impressed with her lately, Nairo has been storming tournaments and she just looks insanely good. She even has the "jank" factor with comboable flip jump kills at stupid percents, and I think flip jump is a candidate for best move in the game anyway. But then yeah... 16 frame grab. It's terrible. It's reactable. And her other options against shield aren't amazing. If her grab was even 12 frames I would say she's top 3 no question and possibly better than Sheik but yeah, that grab is a major major issue.
I really don't know who I'd put in the top 3 alongside Sheik and Sonic. Very hard to decide. I suppose that's a good thing really, it speaks for the balance of the game. In customs I think it becomes a lot clearer that Sheik, Pikachu and Rosalina are the top 3 characters, probably in that order, though it's not totally clear.