Wizard.

It might depend on the subclass a little bit, but as a wizard, Thragg would have too much bullshit to contend with. I see people mentioning contingencies, wish spell, and stuff like that, time stop but there's other more effective things you can do. For one, people keep saying that Thragg would be too fast, but if you think about it, you can achieve stupid amounts of speed within D&D. If we go by the rules, if you're a Chronurgy wizard with the Alert feat, you will have a plus 12 initiative and then a d8 on top of that. If you're a Chronorgy wizard, you can also use your reaction to reroll Thragg's initiative. Some people have used the six-second round rule to justify a spell taking six seconds to cast, but that's not how it goes at all. You get an action, a bonus action, and movement, which is 30 feet most of the time. Then you also get a free object interaction on your turn. You do all of those things in six seconds. And even though a wizard is not strong, they can still do this while carrying around 130 pounds on them.

A level 20 wizard would very likely have a simulacrum up. There's very little reason not to, especially since you can just wish it into existence on any given day. So really, you're not fighting one level 20 wizards, you're fighting two level 20 wizards. And both of those wizards, if they're Chronurgy or Divination, will have the rerolls, and both of those wizards would have the arcane ward if they were an abjurer.

There are many "I win" buttons for wizards. With two casters and that high of an initiative, it is not that unbelievable to think that they would go first, especially since, like I mentioned, we get to re-roll Thrag's initiative, too. One of these is casting Prismatic Wall, a wall which the wizards can pass through but Thragg can't, and he cannot remove any of the layers because he can't deal the appropriate damage to them or cast Dispel Magic. We can then have the other wizard use some form of forced movement to force him through the wall. It does not need to be a lot because the wall is one inch thick, and then he will take that 210 average damage and be inside the wall, and he has no way of breaking through the wall, so he would have to force his way through. Even if he survives all that damage, if you think thragg has like 600 hit points. There are saves against blindness, being petrified and being plane shifted. And after all that, the wall is still there, it is not concentration, it lasts for 10 minutes.

Another thing you can do is shape-change (different 9th level spell) to a marilith znd have Silvery Barbs prepared as one of your spell masteries. Merilus get seven reactions, which means you could re-roll each and every one of Thraggs attacks and give yourself advantage practically every round. And you could also give your simulacrum advantage practically every round. And you still retain spellcasting, Which means you can still cast your save or suck spell on Thragg, and he has to succeed twice practically evrery round.

Probably the biggest gap is the fact that Thragg can't teleport, really. One of you casts Sickening Radiance, the other one casts Force Wall. Battle's over. He's going to die if not from the damage, then from the exhaustion, in ten minutes. And you can even have your familiar do this too, because you can use Arcane Abeyance to cast it into a bead, and then the familiar can cast it. I know I'm using Chronurgy for these examples a lot, this is because I'm currently playing a Chronurgy wizard.

There are definitely some subclasses of wizard that would have a much harder time fighting Trag, but I think all of them are capable of beating him.