"I don't like your plan. It sucks."-Yuna, X-2. I cracked up, because it is so utterly 180 from everything that defined Yuna for so long. Her self sacrificing, her "For the greater good" even in X-2 she's usually at everyone else's beck and call, and finally she just calls out people on their bullshit.
Tied with Aerith's "I can't let you get involved, blah blah blah, that's Cloud's line!"