First off, he's insane skilled as a player, top 5 in terms of neutral and ability to adapt.
Second, he pretty much, mains bayo and secondaries cloud, with that pocket DK. He plays three of the 5 best counterpick characters in the game, which cover any and all matchups. The only characters considered by a majority of people to go even with or win against all three are sheik and diddy, and even then, he's covered by both his skill with his three characters, as well as a possibility of playing sheik or diddy against the opponent. Tweek said after Frostbite he was skilled at essentially the whole cast. While I personally doubt that, it says a lot. Both Nairo and.....Ally? idk, it was two people at the same tournament who cp'ed diddy against salem, oen of which was Nairo. Plus the history of top players, top ten included, of picking up secondaries (Nairo a ton of people, Ally and Mr R Cloud, VoiD's experimenting with zss, larry has a diddy now),,,,,it would not surprise me if Tweek was having trouble against a sheik or diddy and pulled out a sheik or diddy of his own, partially due to his skill, and partially due to how good he is at dittos (for the most part).
Third, opposition. I'm pretty sure the last time Tweek lost to someone outside top 20 was ZeRo saga, quite a while ago. In addition, Tweek is incredibly rough for even the top 20 to take out now. Samsora went game 5, and he lost to Ally and WaDi at the championship, but usually when I think of Tweek losing a set, it's when he fights ZeRo or a few other peopl. So with ZeRo gone, let's take a look at people who have consistently either given him trouble, beaten him, or are just really good and might pose a problem.
Komorikiri: Komo has notoriously bested Tweek in the Cloud ditto, but komo struggles against bayo, and is picking up Roy in response. Because he usually loses the ditto, Tweek goes Bayo, and until Komo can answer this and beat Tweek's bayonetta, he might as well be taken out of the picture.
Dabuz: Not a problem for Tweek usually, but still here for being a top 5 player. Dabuz struggles against Tweek's Cloud, and it's only on those rare days like Frostbite where he pulls through. If he can become as consistent at slaughtering Cloud as he is Bayonetta, he'll become a definite problem for Tweek, and maybe a contender for best in the game. But until he consistently beats Tweek, he's not a problem.
Nairo: Nairo gives Tweek trouble in the past with his Bowser. Now that Tweek has Bayo, I foresee their matches becoming either Nairo's diddy vs Tweek's bayo, or tweek's bayo vs nairo's zss, both of which I see tweek winning. So no real evidence to back it up, except that Tweek is much more proficient with his bayo than nairo with his diddy, and zss suffers against bayo. It all points to Tweek being a problem for Nairo.
Salem: Another example where I'm not really completely sure of what I say because I don't and have not watched their sets. But from general knowledge, Salem's bad at the ditto, and Tweek is good at the ditto. Which would mean Salem would have to really train up his Greninja. So for now I don't see Salem giving Tweek problems.
MKLeo: Possibly the most complex matchup for tweek due to his characters. Tweek has always been Leo's demon, constantly destroying him at the cloud ditto. However, they're proving to be moving past the ditto, which is where things get tricky. Tweek beat Leo's MK with his DK at Leo saga, but Leo might have gotten over that considering he made short work of Konga at Genesis 5. Leo likely would play Marth against Tweek's bayo and beat Tweek there. But Leo also has the pocket corrin, so there's a multitude of matchups to be considered: Tweek's DK vs Corrin, Cloud, and Marth. Tweek's Cloud vs MK, Corrin, and Marth. Tweek's Bayo against MK, Cloud, and Corrin. 9 possible matchups or so. I see their future fights as possibly being giant counterpick battles, each person counterpicking the other with a new character (this probably won't happen and they'll stick to one matchup but it'd be really funny to see). So Leo is definitely a complex MU for Tweek, but I think in the end tweek edges out.
Those are really the only people worth mentioning imo since they'll be top 5 along with Tweek this season, and any close matches or losses he's had from the rest of top 20 can be considered a fluke (considering WaDi got boped by Tweek this weekend, that victory from the championship looks like a bit of a fluke tbh).
In conclusion, Tweek being really good at the game, the characters he plays (as well as the ability to play other characters), and little to no opposition, I think he'll be the best in the world.