I didn't play too many this year, but Professor Layton was definitely the best. Even though I didn't play So Blonde, Sam & Max 2, or A Vampyre Story, given that Professor Layton is one of my favorites EVER then I think it's safe to name it my favorite of 08.
EDIT: Oh, okay, this thread is for PC games. Whoops! Never mind? I don't have a favorite PC adventure game of 08. That seems harsh but none of the ones I got around to playing would I be willing to term a favorite, even if they themselves are their only competition.