"Please Shepard; I'm a Quarian! Give me a hunk of scrap metal, a circuit board, and some element zero, and I'll have it making precision jumps!"
I love Tali (and Quarians in general)
And Joker's awesome too. Anything that comes out of his mouth... though this is a personal favorite.
"Hey, thanks commander! Another dangerous alien on the ship! Why can't you collect coins, or commemorative plates or something!?"
Then there's
Shepard: A little harmless self promotion, EDI. You can let that slide.
Joker: That's it. Go pout. I win.
Shepard: Joker...
Joker: Alright, alright! Good game!
And
"It's just that I'd never pictured the coming overlords to be so... annoying."
And of course
"If you start singing Daisy Bell, I'm outta here!"
Then the stoned Volus...
"I AM A BIOTIC GOD!! I am a great wind that will sweep all before me like... a great wind! A GREAT BIOTIC WIND!!"
And in terms of good, serious bits... I really like how they played out the bit where Tali finds out about her father.
And then a bit that just made me lol...
At the very end of the game, there is a very long fall. Shepard survived, and Tali and Legion were in my squad (as my two favorites) I walked over to Legion and turned him over... the bot was dead. I run over to the tiny Quarian (who has a huge chunk of metal lying on top of her) move it, and guess what! The Quarian survived. The invincible robot can't take as much as the girl who can't leave her suit for her immune system XD