Number one is definitely Achievement Hunter, and the reasons why have mostly been mentioned already. Great cast, with personalities that play off well against each other, lots of running jokes, and ( I think) a good mix of players who are "good", like Ray, Ryan, and Michael, and the "bad (in a funny way)" ones, like Geoff and Gavin. Of course that's not to say each of them can't occupy both categories at any given time.
Other than that, Game Grumps is always fun (and I may be in the minority, but I think Dan is better than Jon, but not by much),, especially with their other shows, and LoadingReadyRun's stuff is good. Which is why I'm incredibly happy they're gonna start doing a ton of LP stuff next year.