Pretty much anything that came out of Jake and Leon's mouth. XD
....Except anything affectionate for Ada.

I guess I should be more specific.
Jake: "Better put a leash on that puppy."
Leon: "Sherry? What are you doing here?" He just sounded so concerned and caring. And I'll never forget when I heard Leon call the blonde mystery girl Sherry in the trailer confirming who she was.
Chris: "GET THE HELL OUTTA THERE!!! LEON, LEON?! ARE YOU ALRIGHT?!!" I also love this line, not just the words by themselves but the tone and stress in Chris' voice. For so long I knew Chris and Leon were most likely friends, but to see it semi-confirmed in the games by showing how he cares about Leon. (In just a friendly way of course

) Chris really sounded devastated when he thought Leon was killed in the explosion...Well, he was the way with Finn too, but I don't care Leon and Chris are best budds I tell ya! Naner naner! Ahem, enough rambling.
Chris: "Fill her up!" XD That kinda brought back memories cause it reminds me of some cheesy 90s dialog like the old REs.
Jake: "You saved me, you know that?...Thankyou."
Piers: "What the hell were you thinkin' pullin' a kamakaze stunt like that?!"
Piers: "Grrrr." No seriously, I loved it when he growled at Helena.
The entire argument between Jake and Piers if they jump on the side escalator together.
How bout the worst line? = "I lowered my asking price to $50." What the phuck Jake?! They told you yes without a fight, why not just ask for a million at least?! You know what? You are loose in the noggin like you father.
I have more, but I'm done for now.
Edit: Oh crap, I misread your title. Favorite LINE as in single. I went overboard, so sorry. I've been very talkative about Resident Evil lately since I just got my hands on 6.