I voted purely based on which one I enjoy watching on TV the most. Too me the WSB coverage can't be beat....This weekend I watched all three series', and the WSB round at Kylami(sp?) had me glued to the TV. I even taped it a watched it again the next day. I find it's really well produced, and the way they use the on-bike cameras and everything, just makes for exciting TV.
I really don't know which I would say is my favourite series, (I'm a Canadian Superbike fan too, and I follow the BSB a bit), but WSB makes for the best TV IMHO.