Wheatley, though obviously whoever said he should have been voiced by Ricky Gervais is wrong. Brilliantly funny and wonderfully scary in a maniacal way towards the end of the game.
GLaDOS, of course.
Old Aperture Science. I love stuff like that, exploring the old stuff, getting glimpses of the background story with each step. And of course, the biggest highlight of the game:
Cave freakin' Johnson. So. Many. Lines. "In case you get covered in the repulsion gel, I have a note from the boys in the lab... here it is: Do not get covered in the repulsion gel. We haven't quite nailed down which element it is yet, but it's a lively one! And it does not like the human skeleton." "That's not part of the test. That's asbestos."
The ending. Portal to the moon. Such an awesome scene. Just awe-inspiring. Wheatley's desparate attempt to save himself, GLaDOS' change of heart, then deletion of that change of heart. And the opera! Oh god I am in love with that opera. Really quite moving. Which brings me to the one downside of the ending, which is the song during the credits. Pretty crappy. Should have just skipped it, it was funny (and very good) in the first game, but now it just seemed out of place and tacked on.