It's very difficult to decide, partly because there's so many videos and partly because there's so many great moments scattered throughout. But for what I can recall now, I think it would be his review of Tomb Raider: Underworld.
Not sure why, but it's one of those videos I recall when I think of Zero Punctuation. And honestly, the Lara Croft + Jason Vorehees-marriage concept was spot on. She isn't healthy, that lass.
As for worst... None is really bad, barring the holiday specials with no content. If I have to settle for one, it would be Bioshock. Still a good review, but it's certainly not one of his best. It feels rather disjointed and most of the criticism seems rather forced, or poorly elaborated. But again, it was one of his earlier ones, so I suppose that explains it.
Of course, before I figured out how reviews worked, I'd consider the ones where he (rightly) disemboweled one of my beloved games terribad sharkjumping moments, but I've grown a more functional brain since.