Portal 2 is a game full of hilarious writing from a variety of characters. I'd vouch it was better than Portal with the inclusion of Wheatley and Cave Johnson.

Cave Johnson is the funniest character, in my opinion. He's insane (in a good way) while similarly charismatic.

"Alright, I've been thinking. When life gives you lemons, don't make lemonade. Make life take the lemons back! Get mad! I don't want your damn lemons! What am I supposed to do with these?! Demand to see life's manager! Make life rue the day it thought it could give Cave Johnson lemons! Do you know who I am? I'm the man who's gonna burn your house down! With the lemons! I'm gonna get my engineers to invent a combustible lemon that burns your house down!"

What characters and scenes were most memorable to you?

"We've both said a lot of things that we're going to regret. But I think we can put our differences behind us. For science. You monster."

"I'M DIFFERENT"

"Holmes VS Moriarty. Aristotle VS MASHY SPIKE PLATE!"

"So. How are you holding up? BECAUSE I'M A POTATO"

But
How can I possibly pick a favorite..
Wait, can I pick the whole script as my favorite? I pick that. :B

So many I can't even choose. But you see I have a shortish version of the lemon one in my signature

I think the posters are awesome. And generally Glados says the most funny things.

"Look at you. Soaring through the air, so majestically. Like an eagle; piloting a blimp..."
"Now you're thinking with stupidity. Now you're just not thinking..."
"You are the type of showoff who only shows off really stupid things.."
There are others, but Its so hard to choose