DA:O/A: Oghren, just because he was such a disappointment. When I met Oghren, I thought he'd be a clever subversion of the usual Merry Drinking Dwarf stereotype. After all, he had a real reason to turn to drink, he actually seemed to be hiding a deeper depression, he had suffered consequences in polite Orzammar society for his behavior, and so on. Turns out, after you're done in Orzammar, he's basically just that stereotype again. Come Awakenings, and whatever little subtlety he had is completely lost and he's a cartoon character.
An then there was that elf lady from Awakenings. I seriously have not bothered enough with her to remember her name. Velanna? I don't think I've ever met anyone who's tried to defend her, either.
DA2: I dislike Sebastian, but I dislike him for what he is, not for how he's written (as in, I don't think he's badly written), so that's fine with me. Tallis is really the only character I couldn't stand. I'd never heard of Felicia Day until I googled the DLC after the fact to try and understand what the reason for this weird character was, so maybe it helps if you're a fan of her? I just found the whole plot constructed around her very flimsy, the fact that you couldn't really argue with her a cop-out, and her cutscene competence a little overmuch. Thanks to Talli, MotA did feel like playing someone's self-insert fanfiction - and it wasn't even particularly good self-insert fanfiction.
DA:I: I actually don't dislike any of them. I've never really warmed to Leliana, not here or in DA:O, but since I can't really put my finger on why, I can't defend that position, I just... don't click with her. Sera is grating sometimes, but I feel like she's written to be that way and since Varric is glued to all my Inquisitors' sides, I really rarely see enough of her to get an "overdose" of her banter.
I think Dorian comes the closest. He's at his best for me whenever his political stuff from Tevinter carries over - while I vehemently disagree about hist stance on slaves, for example, I find it interesting that he holds these views -, but his personal quest with his daddy must've been cut wholecloth from some early-2000s-YA novel, with a little blood magic thrown in for flavour. Basically the most clichéd plot you could settle a gay character with, and no one in the writing department tried to do anything remotely special with it. Worse, since I basically knew this confrontation scene with his dad from a dozen shows and books made for teenagers, when my Inquisitor romanced him and that scene started, it felt like Quizzie was dating a 15 year old to me, haha. I suppose Dorian is very young (like 19-22 at the highest? That's what he came across like to me), so it's fair enough, and I know some people found his story inspiring, which I honestly think is great, but it just bored me. And don't get me started on AdoriBull. I don't care whether it's abusive or whatever, I don't think it is, I just care how unbelievably cringeworthy their dialogue is, and I find the whole qunari-tevinter-forbidden-love angle, again, clichéd and boring. It just highlights for me all the ways in which Dorian is already a very standard character who didn't have much to offer me, even after I forced my Inquisitor through the romance with him in hopes that it would make me like him better; and it also underlines IB's most annoying traits. It's the first time a companion romance has made me actively like them both less.
In Trespasser, without wanting to spoil, I actually really enjoyed Dorian and wanted to see more of him. I also do like his banter, so I couldn't even say I don't like him. However, if neither he or IB is romanced, I keep them apart like gasoline and a match in a fireworks factory. Not once in the same party. Not. Even. Once.
Edit: Oh, lest we forget: Bianca. Special mention because she's not a companion and barely around long enough, but what a letdown that was. One and a half games of build up for this stupid crossbow, and then it turns out it's all about some trite lovestory with a completely unlikeable character that they desperately tried to make cool in some way (no really, she's totally ten times as talented as Branka!). Needless to say, it felt very good to part Varric from his bow on my second playthrough and make him into a badass knife-and-dagger dwarfy, Bianca forgotten in his inventory.