I don't personally have a favorite, though I did enjoy this one, where a working therapist played Eliza and reacted to all the bogus AI therapy the game makes you do.

I asked Zach if he has any, and the one that came to mind for him was an old-school forum style Let's Play of SHENZHEN I/O, completely in character as the engineer you play in the game, with lots of explanations and interaction from other posters. It's called (Warning: Something Awful link) Shenzhen I/O: The Engineer's Blog, and it's also archived here if you just want the main thread (and/or don't want to go to SA): SHENZHEN I/O by Quackles