Tali. I know she's uber-popular and stuff, but her character has always really bugged me because as far as I can see, SHE DOESN'T HAVE ONE. Now, I like the Quarians. I think they have an interesting history, their whole biological dilemma is intriguing, and they have a nice political situation going with the Geth. Cool, I'm interested. But it seems like everything that Tali ever talking about is being a Quarian. What the flotilla is like, what it's like being in an exo suit all the time, what a Pilgrimage is like. These are all interesting, but these are Quarian things, not Tali things. It's like Tali's defining characteristic is that she is a Quarian, and I never really got the sense that her character much depth beyond that. Wrex and Garrus both seemed to have a real history, they actually seemed to be real people. Wrex talked about the Krogan and their history quite a bit, but he actually had views and opinions, while Tali just talks about what it's like being a Quarian. Liara showed shades of what I've been talking about, but the difference here is that she actually has interests beyond being an Asari, and talking about them. She has more than a passing interest in the Protheans, seemed to have an interest in humans, and has been studying and been isolated from the world these last few years, which is quite different than the vast majority of Asari her age. Her character is a bit weak, but at least it goes beyond "the Asari squadmate."
What I guess really annoys me with Tali, is how she's forced on you, and then never does anything. Garrus and Wrex don't really do much for the plot after they're recruited, but they are optional. Tali is not, yet after you recruit this teenager, she never does anything for the plot again. Maybe she makes a few comments in the comm room, but for a required squadmate, Bioware couldn't have given her anything to do besides give you exposition on the Quarians? Her capabilities aren't convincing either. She's apparently some Engineer prodigy, but besides a few comments about how good she is with the ship (which, again, is more of a Quarian thing, the knack for these kinds of things), I never really got that impression. Kaidan and Ashley are career soldiers, Wrex is an experienced mercenary, and we see Garrus demonstrate his stealth and aim when he rescues Michel. Tali...detonated something in the alley I think? Been a while since I've played, but it didn't really have the desired impact if that little scene's purpose was to demonstrate her engineering aptitude. This is a problem, I think, because she really needs a scene to establish her as a badass, or demonstrate that she's exceptional with machines, or SOMETHING.
A bit long winded, but anyway, Tali's character has always just bugged me, and I really don't see why people just get so rabid about her. I've just always felt her character has nothing going for except for the fact that she's a Quarian. I guess she's mentions her father and his position every once in a while, but I never really got a feel for what made Tali interesting AMONGST Quarians. I could forgive this if she felt like the Engineer prodigy she supposedly is, but I never really got the impression beyond other characters praising her or the codex that this was the case. Then she's handpicked by TIM as a prime candidate for recruitment, and she still just feels off. The very least we could've seen is a little cutscene of her demolishing a few Geth, but we don't even see that. Overall, she is the "Quarian squadmate" and her character to me never really takes root beyond that.