IMO, this question has a lot more complexity then we have given it. The most potent strain IMO, is too hard to say because different people react differently to different chemical compisitions, meaning that for some an indica is going to seems very overpowering in a narcotic type of a way, while for others sativas may just be too trippy for there liking.
I have not grown Nirvana's widow, but I have grown sensi's white label white widow, and it was some major smoke, but i have to say that it had nothing on the chem dog bx that I picked up from rez.
White widow is to Amsterdam what Kush is too Cali. My all-time favorite white strain is White Russian from Serious, that is some very seriously powerful stuff