ZSS is overrated; I predict that she'll eventually fall to the bottom half of the high tier characters. Paralyzer/B-reverse paralyzer is great, and spaced n-airs are great, but everything else isn't really that impressive. She does have a strong dash attack, but it can be punished and relatively it isn't that great because everyone in this game was given a strong dash and a strong jab. Characters like Pikachu and Peach can actually *kill* with their dash attacks, which ZSS really wishes that she could do, because her kill power is one of her biggest problems. The side-b no longer kills, d-smash doesn't link into instant death any more, up-b can be punished extremely hard and smaller characters can pop out of it if they get hit while high up, b-air goes over smaller characters entirely and is difficult to land anyway. F-smash is surprisingly good: it can be punished because it's two hits, but it has great strength and startup speed. Her other viable option is grab to up-air, but landing ZSS' grab can be very difficult.
Which is another issue! The character's main weakness from Brawl has been mitigated (tether grabs are better; her other options are better) but not removed--she still has that long grab animation, making grabs very high-risk, low-reward for her in general. Shield beats most of her low-risk options easily; she can condition her opponent into shielding and then start to grab, but once she starts to grab, her opponent can just counter-condition and punish those whiffed grabs very hard.
It's kind of unfortunate that otherwise good characters like Pacman and ZSS are stuck with bad grab animations (Pacman's in particular is egregiously unfortunate, because he has big gloves and didn't even need to have a long grab). I'd support both for top tier if their grab animations were normal. (some others like Palutena suffer from unnaturally slow grab animations too, without even having tether grabs--really bizarre)
The characters in this game who will end up being at the top of the tier list are those who can net kills most easily while being good in other areas such as speed/spacing. Because of the Rage effect and how long stocks tend to last in this game, closing a stock gap often involves taking significant damage, so edgeguarding and/or landing easy kills/confirms is critical.
Kill throws are very important too, since shield is strong in this game. The best way to rip off stocks is to have a kill throw because your opponent can't just shield your smashes if you can grab and kill at high %.
Diddy Kong is a very strong contender for top tier because his kill options are ridiculous.
d-throw to u-air is very potent--you can vector, but you will get hit by a frame trap most of the time--and u-air kills very early (his dash grab has a little less range I think than it had in Brawl, but it's very quick, and his ground speed is good as always). At higher percents, Diddy's b-throw and f-throw can kill at the edge too. If he can't kill with his regular throws, monkey flip also kills--an aerial command grab that kills and starts from a distance away is amazing, and it can even be smash-attacked to give it extra range and strength.
His f-smash and f-air have surprisingly enormous range, and his up-smash and up-tilt (!) actually kill in this game, so this character has no shortage of non-grab kill options. Bananas are awesome for setting up kills, too, and if you use the Battering Banana opponents actually get launched *up* when landing on a banana, which makes landing an up-air or f-air easy. The nerf to airdodge makes all of this even better because if you airdodge any of these moves, Diddy is just going to fastfall f-smash you with that range (or u-smash/u-tilt for mixup), or side-b if you're far enough away.
Lucario is another obvious contender for top tier. He kills way too early once he gets to a higher percent (80 to 120). Not much to say about him: almost every single move he has can kill at some percent. Even Extremespeed.
Rosalina kills fairly early with that f-smash and up-smash with Luma, but she's top tier for other reasons instead--namely that range and all those hitboxes (there are 4 on her d-smash). Still, she might struggle against characters like Sonic who can rush her down, because she's 1) very tall, 2) very floaty, and 3) very light, not a great combination. I believe her b-throw can kill, though (lots of strong b-throws in this game).
Sheik is great, but does have some issues killing, and I think that might cut her from the very top of the list over time (or maybe not: it depends on how her matchups look). Confident use of bouncing fish (e.g., f-throw to bouncing fish) and great edgeguarding are the hallmarks of a top Sheik, because that's the best way to net kills. You can also use Vanish as a mixup to secure a kill. Up-air can kill too. So there are options. Whether or not that's enough, time will tell.
PIkachu might be a surprise top tier--his edgeguarding is pretty much the best in the game. That multi-hit bair is incredible for edgeguarding now that airdodge has been nerfed, and up-air/f-air/n-air is as good as always. When a character is on the ledge you can run off and rising b-air back to the stage to cover the get-up; up-tilt and up-air will cover the ledge jump option. Thunder is now a ridiculously good spike option offstage. Outside of edgeguarding, fastfall f-air to pretty much anything can also net kills. Only thing is that Pikachu's throws aren't that great for killing iirc; if he had a good kill throw he'd be top tier for sure.
Sonic is looking to be pretty top tier. His b-throw kills even earlier than Lucario's until like 150% (when Lucario becomes a beast), and he's fast enough to get the grab most of the time. His f-smash doesn't have a whole lot of lag either, and his edgeguarding is great because of that semi-invincible up-b (you can d-air off stage and still make it back). Combined with the rest of his game, his kill potential probably makes him top 5.
Ness is really good. That b-throw kills *so* early, and it's paired with one of the fastest dash grabs in the game (though Ness' general ground mobility isn't that great). Aerials are amazing too; probably the best aerials in the game (n-air out of shield is awesome, f-air has tons of range, up-air has a HUGE MASSIVE HITBOX HOW DOES NESS' HEAD GET THAT BIG OMG WHAT THE HECK?). Ness' recovery makes me a little reserved about him, though, because Rosalina just eats it up, Dark Pit/Pit can reflector it, etc.
I think Yoshi and Greninja are worse than the above because of fewer reliable kill options (though Greninja up-smash is amazing), though they've got tools for pretty much every situation, so time will tell.