My first yoga -- that is, the stuff I've repeated more than once and later felt some improvement in my ability to do it -- was P90X.
Later, a friend told me, "Tony doesn't know anything about yoga! What he's doing is too damn hard." Eh, maybe, but that's the one I've done the most.
Tip: My first time doing yoga (not P90X), I used one of those thin yoga mats on the gym's hardwood floor. I might as well not have had any padding -- my knees hurt so much that I couldn't focus on any of the poses. I laid down two mats the next time and started enjoying it.