As others have said, Red vs Blue is the one to watch.
It pretty much made machinima what it is today; it was the first significant machinima success, and has helped a lot of others get started because of its success and influence. It's not difficult to see why, either: it's well-written and edited, and it's really, really funny. The earlier episodes are, in my opinion, not quite as good as the later ones, but from Caboose's first drop in intelligence, it's comedy gold.
They've started a new series fairly recently, actually: RvB Recreation. I wouldn't recommend that you start by watching that, though. To understand what's going on you really need to have seen all of The Blood Gulch Chronicles (the main series), the Out of Time and Recovery One mini-series, and the follow-up to the Chronicles, Reconstruction. There's a lot of it, I know, but it's all great stuff, even if the plot does get a little hard to follow around season 3 of the Chronicles.