Best does not mean fastest. Fastest means fastest and in that case, the X Series, if driven well, are the fastest. Best however, is a competely different story and a catagory in which the X series doesnt fit in, in my opinion anyway. Even "Best around a track" cant include the X series as its a matter of personal opinion, and most the people I know despise driving the X series for the sheer fact that you need reaction times so fast its ridiculous. So, use Fastest instead of Best to save some confusion.
As for the Fastest, as said earlier, it all depends on Driving style, tune, who's driving and many other variables. For me, the X2011 is faster, second is the Vettel, then the 2010 Prototype followed by the Coloured that you buy in the dealership. But I have tuned the X2011 more than the others.