Tali > Mordin > Kasumi is probably my top three right there.
Tali's is just awesome from story AND gameplay aspects, the dialogue options are superb, and the final choice is one of the toughest to make in the game, if you don't pass the paragon/renegade check. And the aforementioned quip from Shepard about Tali's exile.
Mordin's has a similar tough decision to make at the end, but I don't find the krogan hospital to be nearly as appealing a locale as the quarian ship was. However, with no disrespect to Tali, I enjoyed debating the ethics of the genophage with Mordin a hell of a lot more than discovering what Tali's father was up to, and having to decide the fate of the quarians. Sad, but true.
Kasumi's was actually kinda boring at parts, though I was pretty excited the one time you got to shove 3 mercs off a huge ledge at once. That was pretty fun. Finding the various works of art in the vault was pretty neat, too, as was the grand escape. I wasn't too big a fan of the final boss, but it was OK. Really, the big reason it's in my top 3 is the discovery of the SMG in the vault. That thing's probably my favorite weapon in the whole game.
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Xbox GT/PSN name: TatteredUniform
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