"We impose order on the chaos of organic evolution. You exist because we allow it, and you will end because we demand it."Zakarath said:"You exist because we allow it.
And you will end because we demand it."
(or something like that, I probably messed it up a little)
Also:
"I'm Commander Shepard, and this is my favorite store on the Citadel."
That's the whole quote, and yes, it is the best one. That's like SHODAN levels of machine evil.
Runners-up: "Conrad, I haven't been shot in the head nearly enough times for that to sound like a good idea."
Responding to Liara's observation that they're walking into a trap: "Your grasp of the obvious never ceases to amaze me."
Mordin: "Bit of a cloaca."