From ME1: Liara and Ashley. Liara may have had potential, but she was written as THE alien space-babe cliché. "Ooo, Shepard, you're so fascinating, also, did I mention I'm a virgin?" It's especially bad during the romantic confrontation scene: if you do suggest a threesome, she doesn't bat an eyelid and reacts as if you'd chosen her. Apart from her almost instantaneous attraction to Shepard and her role as a "walking Asari encyclopaedia", she literally had NO conversation in her and not an ounce of humour. "I'm afraid I'm not very interesting". You got that right. And I don't care that she saved Shepard's life, that still doesn't make her any more interesting, even her "darker" ME2 side. That just creeps me out even more.
As for Ashley, well, somebody else already summed it up perfectly. "I can't tell the aliens from the animals". I'm sorry, what? I don't care that she supposedly changes her outlook later, but that line immediately killed any kind of sympathy I may have had for her. She made an explosive addition to the Virmire scenery on every one of my playthroughs. In a way, it actually complements her role in the story: the tragic sacrifice is a fitting conclusion to her "army reject" storyline.
From ME2: Jacob...Not only is his personality hopelessly bland, but he acts like a total jackass to Tali ("introduce yourself to the ship's AI"). I guess I can understand his misgivings about Thane, but that didn't help to make him any more likeable either. He also completely fails to grab my interest as a potential LI: I don't find him attractive (those lips seem to have a life of their own...), and his skin-tight uniform with the overly blatant codpiece disturbs me. Fail on all fronts.
Miranda's probably not a bad character objectively, since, as people have said before, she's meant to provoke strong reactions, but I really can't stand her attitude. Yes, her ass deserves an Oscar, and yes, she did bring Shepard back, but that's about it, and I'd really rather have not been indebted to her for that. I'm annoyed that the game tries so hard to shove her down your throat. I'd rather find ways to replace her skills than having to put up with bringing her along on missions. Also, "I'm so perfect, and it bums me out"? Boo-frickin'-hoo.
Grunt isn't much of a character at all, but at least he provides some entertainment: that's more than you can say for either of the above. And while Jack's attitude is well-nigh infuriating, I find her reasons easier to stomach than Miranda's. You really can't expect someone to be a functional person after being put through all the stuff she's been put through. I also find her face to be much more attractive than Miranda's, even with the shaved head.