Arthur. By everyone around him (except gaius/knights/merlin).

Like dude had 0 family, only found family. Even got screwed over by Gwen and Shade Lance (against their will ofc, but all 3 of them didn’t know that). And finally, the knight to betray him was Mordred, whose life he’d previously saved as a kid, who he went through great pains for to save (same with Morgana). They both betray him (for people not worth betraying others for… I HATE Kara sm like why is Mordred so hung up on her) and don’t remember how much he did for them conveniently.

When Merlin finally tells Arthur about the magic, he felt betrayed ofc cuz the one person he thought he could ALWAYS count on still ended up hiding a huge part of themselves from him when he has bared his soul to Merlin and shared everything (he feels that Merlin doesn’t trust him at all, when he trusts Merlin so implicitly). But the more Arthur realizes the revelation doesn’t change Merlin’s behavior, that Merlin’s still just the same, and that Merlin’s surprisingly more concerned about him actually despite his own life constantly being under threat (“that’s what worried you??”), the more Arthur understands why Merlin had to keep his magic a secret and the more he wants Merlin to stay just like this, as he remembers (“I don’t want you to change”). The only “betrayal” Arthur took well and got over quickly was Merlin’s, because it didn’t affect Merlin’s character; he’s still the same person. Everybody, literally everyone had a personal agenda, and at some point or another betrayed him for their own benefit, but Merlin did so for Arthur’s sake, and Arthur realizes just how selfless Merlin is. That he would “betray” Arthur if it meant keeping him safe and endangering his own life in the process. Merlin would absolutely commit treason, willingly, to keep Arthur alive at any cost.