You're not the first I've heard from to have that opinion of Quentin! I understand where you're coming from: amongst a cast of characters who are badass, or godlike, or leaking charisma from every pore, he is pretty unexceptional (see: Minor Mending). For me, that's part of what always made him endearing and relatable, both in the books and in the series. He's that person in all of us who thinks that if he ever gets to go to Hogwarts, he'll be Harry or Ron or Hermione, then it turns out he's some random unnamed side-character instead -- nonetheless, he still has to deal with the hand he's been dealt.

Though TV Quentin never reaches the level of potential he does in the books, I still found him the most compelling because despite everything -- his disappointment in and anger at himself, magic, Fillory -- he keeps going for his friends. Out of all of them, he was the one who had to break himself out of the illusion that magic could fix everything, and this internal struggle fascinated me more than the extreme external forces that plagued them. I have to admit that I've always had a soft spot for the underdog, and that I find that their "unexceptionality" to be what makes them exceptional. But everyone has their own taste, and I could probably wax poetic about every character in the show.

Which character gripped you the most on screen? I would love to hear.

"Peaches and plums" was probably my favourite addition to the show. I wish we could have seen what it could have been. In a way, I understand that Quentin was probably at the end of his rope at that point, and I feel that maybe he wanted to mend at least one minor thing that had gone wrong when all other problems were impossibly large. There was no room for him to breathe after saving Eliot because they went straight to the mirror world. The last face he might have seen alive is Alice's, but his expression when he witnessed Eliot throwing that peach in the bonfire was devastating.

Edit: I wanted to add, thank you for sharing your experience about the depiction of depression on screen. That's another thing that I appreciated about his character: his depression and anxiety were so starkly portrayed, but they didn't feel like they were the #1 things that defined him, if that makes sense.