Alrighty so I've played quite a many games by now, so I have some thoughts on the mons I'd like to share!
Regieleki
There's been a lot of discussion on Eleki, and it's entirely unsurprising as to why. At first, it appeared like a miserable Electric wallbreaker with a blazing speed tier and nothing more, but then we got the data on Transistor, which in tandem with its speed turns Regieleki into arguably one of if not the best revenge killers in the metagame, and an extremely potent wincon in the right hands.
At first, I experimented with a mixed offenses set with Life Orb, which would let it clean games against chipped Ground-types as opposed to necessitating the Ground-type's outright removal first, also allowing for it to revenge kill Pheromosa. While this set certainly works, I've found out quickly how amazing
Choice Specs is, especially when paired with Rising Voltage. Although this means outright removal of Ground-types before laying out the foe, it means that Regieleki can win the game on the spot with its nuclear attacks and entirely unrivaled speed tier able to outspeed a majority of Choice Scarf users. Although its movepool is unbearably linear, its power and speed more than make up for it, and have made Regieleki a major surprise for me coming into the tier. I've managed to rack up a lot of elo on the ladder using Eleki as a consistent wincon, and especially with Rising Voltage, it has not at all disappointed.
Its success is a likely surprise to many people, but after using it plenty and thinking on it, I believe the reason behind Eleki's success in execution, despite the severe movepool setback, is because of how incredible it is at forcing in Ground-types, and how the Eleki user can capitalize on that to get in another breaker such as Kyurem-Black, Genesect, Pheromosa, or Spectrier. In combination with hazard damage and the ability to apply major pressure with the help of Eleki's mere existence, wearing down the foe's Eleki check(s) becomes a pretty easy task, and it almost always is a wincon no matter the matchup.
I imagine even if Eleki falls out with the loss of its broken partners, that it will always have a niche in the tier. It's not at all banworthy (at least at the moment), but it's a pretty strong Pokemon in the right hands, and as Gross Sweep mentioned, it is amazing for teaching new players how to detect a wincon and how to get to the point of winning one.
Genesect
Genesect is, without a doubt, the best Pokemon in the tier; more specifically, with its CB Extreme Speed set. I would have said Eleki was the best revenge killer if it wasn't for the existence of Genesect, but Genesect has completely taken the tier by storm, and to nobody's surprise! I won't repeat what everybody else has said, but it is beyond broken. All of its sets are amazing, and it can immaculately prep for literally any matchup it wants, and even if it has blind spots, it can just U-turn and sweep games with CB ESpeed in the late game. It's fucking insane, and is definitely my top suspect for a quickban currently.
Pheromosa
Pheromosa admittedly underwhelms me. I'm not sure if it's the omnipresence of Choice Scarfers, Regieleki, and Genesect outclassing it that's keeping it in check, but I find myself being able to handle it with surprising ease. I can certainly say that it has the potential to be broken, but it can only really get away with HDB in this metagame, and its a lot easier to telegraph and punish than in the past generation; it doesn't help Z-moves aren't around for it anymore to snowball, either. To be honest, I think it has the potential to stay, but only time will truly tell.
Naganadel
Although Z-Draco Meteor isn't around to let it retain its obscene snowballing as far as offensive boosts go, it is extremely good at taking advantage of passive Pokemon and can plow through teams really easily as long as it gets one kill thanks to Beast Boost letting it outspeed literally anything. Although it struggles a lot against offensive teams, it really only needs one kill and Heatran to be gone to really get going, and even in such a speedy meta, I can't see it staying with how absurd it is offensively.
Landorus-I
Landorus-I is an amazing offensive Rocker and stop to Regieleki, and its access to U-turn to bat really helps it out in the Corviknight/Skarmory matchups. Earth Power + Sludge Wave gives it a lot of coverage as far as breaking goes, though I've seen the occasional Psychic as well. I think it is certainly a banworthy threat, but it has a pretty hard time in such a fast meta as it is; I feel it's like Pheromosa where it will probably face justice later when the metagame begins to actually develop.
Though, Zneon made an interesting observation in the form of Gravity, which I've seen to a pretty common degree as well, and it works amazingly, letting the combination of Earth Power + Sludge Wave go wholly unchecked. I personally think it's a bit more consistent as a Stealth Rock setter due to how amazing it is at forcing switches and laying out pressure with its presence alone, but Gravity certainly has a pretty interesting application to the metagame, and is something that shouldn't go without consideration. Overall, it's pretty good, though I can see it leaving in a later quickban round, or possibly a suspect.
Swampert
Swampert is a complete surprise to me, but a very pleasant, welcome one! Flip Turn was a
perfect addition to its arsenal, and I mean genuinely perfect. Its bulk and typing by its base is really strong in the metagame, but Flip Turn gives it an incredibly important, massive tool that lets it keep up with such the offensive, momentum-oriented state of everything, and one that hits important targets such as Landorus-I/T super effectively, no less. For this reason, it partners great with Regieleki for its ability to effectively chip down Ground-type targets, dealing with opposing Regielekis, and setting Rocks for its offensive teammates. I expected it to fall to the wayside, but it has a genuine niche right now thanks to the unexpected addition of Flip Turn, and one that I find fairly strong right now.
Spectrier
Spectrier has a similar problem to Regieleki, but with a much more fearsome STAB to bat in Ghost and utility options like Taunt and Wisp to combat its shit matchups. Although Dark-types and Normal-types put a very easy stop to it, its ability to snowball is unlike anything else in the tier thanks to how spammable its Shadow Balls are in general. I find that Nasty Plot + Taunt + 2 Attacks to be its best set, though Choice Scarf is also incredible, and can take games on the spot against more offensive teams.
As the metagame develops, I can see Hex growing in prevalence too, and I think eventually Choice Scarf will become its pinnacle set. Though, its power and speed tier really lets it stand out, even against Dragapult. I can see this thing staying, in all honesty.
Zygarde
Zygarde isn't broken whatsoever, but it is fairly anti-meta against Regieleki builds, being able to spread Glares, set Substitutes, and set-up Dragon Dances to then throw out Thousand Arrows. Almost all Zygardes I see run Glare, which I feel makes complete sense, and works to a pretty egregious extent against offensive teams. Zygarde's in the group of Pokemon that I can see eventually becoming problematic, though--but at the moment, it's pretty palatable and has a pretty neat anti-meta niche.
Landorus-T
Lando is just as good a glue as it was past generation, though I find its defensive Rocky Helmet Stealth Rocks set to be by far its best variant. It's really good at taking on U-turn spam and Melmetal, spreading great chip damage and helping to open up fellow breakers for Bulky Offense and Volt-Turn alike.
Blaziken
It's really hit or miss in such a fast metagame, but another Pokemon that has the capacity to completely claim games in the right hands, similar to Naganadel. I think its cleaning ability is a bit too much, especially with how good it is at taking advantage of passive builds, but it certainly didn't perform as consistently as I expected it to. I still expect it to be gone pretty swiftly, though, and for a good reason.
Kyurem-B
broken broken broken broken broken broken broken broken especially with regi broken broken broken broken Thanks for coming to my ted talk!
In all seriousness, Kyurem-B's breaking power, especially with Dragon Dance, is beyond absurd. It's way, way too good at opening holes and taking advantage of the brutally fast, power crept nature of the metagame with forced switches; in fact, I find it to be Eleki's best partner, as Eleki forcing a Ground-type in is complete free real estate for Kyurem-B to either set up a DD and start breaking major holes in the opponent, or just click a move and get major chip damage on the foe to the point where Eleki or other faster offensive components can just wipe games. tl;dr: fucking absurd mon, as to be expected.
Cinderace
Unexpectedly, I have seldom seen Cinderace on the ladder. I've seen a few, but not enough to make any solid conclusions on it. From my observations, though: it has a rough time. It can't afford to run HDB due to it having the inability to boost its speed naturally, and it has a really awkward speed tier as a result, meaning it really wants a Scarf to keep up with everything, but when Stealth Rocks are everywhere, chip damage racks real fast. I've seen Sucker Punch used a lot too, which certainly helps out, but it's a lot less fearsome than I expected.
Zapdos-Galar
Zapdos is an awesome Choice Scarf user, with amazing dual STABs, U-turn, and access to Stomping Tantrum to batter the Koko matchup. It's super good at keeping hazards up, as a Defiant boost can spell major breaking against the opponent. Even without Defiant, though, the threat of its STABs alone with its base 125 attack is crazy nice. Even if Zapdos falls out of fashion, I think it'll always have a niche in the tier.
Anyway, that's about all I got for now. I have a lot more thoughts, but I just wanted to give the preliminary stuff as far as expected threats and surprising picks go. Thanks for reading!