Tali steals the show in Mass Effect 2 with the best combo of recruitment + loyalty quests (rivaled only by Mordin, maybe). ME1 is Liara's, though. Of the six teammates, she is the most pivotal to the main plot, and as one of only three love interests, she gets the most dialogue of anyone besides Kaiden and Ashley, and who'd wanna speak to them anyway. Tali might actually have the fewest lines of the squad, and her importance to the plot ends as soon as they leave the Citadel.
Liara is naive in the first game, but it just serves to make her growth throughout the series that much more compelling and, in my opinion, satisfying. Liara really was just a brilliant archaeologist forced to adapt to a frightening new world, and she did it like a boss.
As for ME3, Liara was with Shep from the start, whereas Tali doesn't even appear until like 70% through the game, and she and her peeps are outclassed by Legion and the geth when they do show up. For the leading ladies, I'd say it GOES
ME1: Liara
ME2: Tali
ME3: Liara
They are absolutely the top two female teammates, though. Jack's character had wonderful closure in ME3, but she doesn't compete with these two. Kasumi might have been the best ME2 character OF ALL, but she got the royal shaft in the last time, and sorry Leon, but I think most fans would agree that Ms. Lawson isn't even in the discussion.
As for the male squaddies, I'd say the only guy who can even be mentioned alongside Garrus is Mordin. Legion is also amazing, but he shows up too late in both games, and his big moment isn't as powerful as Mordin's.
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