Possibility of being clean I would say
Quintana 5% - that is the probability that a clean rider could acheive the insane outputs he has done, while opting to ride for a doped team like Movistar AND that his habbit of disapearing for months at a time prior to gts is totally innocent.
Nibali - 1% - that is the combined probability that
A)he never did visit Ferrari
B) that Astana was doping all their other riders but not him and yet he still was the best,
C) that Liquigas was doping all their other riders but not him and yet he still was the best
D) that a clean rider would become bffs with Basso while Basso was doping
E) That his slightly less unbelievable performances are clean
Froome - 0.00001%. Where to begin. First of all the probability that every single time Sky and Froome lied about something, itwas purely an accident slip of the toungue and they never really meant any of those things. That includes Bilharzia, lies about training 10 hours a day and not feeeling any fatigue, lies about previous performances being ridden on poor equipment when that clearly was not the case, lies about kerrison inventing new time trialing science for Froome that the actual cycling experts never thought of etc.
On top of that, we can add the probability that someone could actually peak for 6 straight months, that they could beat Armstrongs mountain and time trial performances.
The probabilty that someone could lose 10kg naturally, without losing any power without drugs.
Also the probabiluty that someone who did beat all of Armstrongs performances would react by being surprised they were under suspicion, and plain out refuse to share meaningless data like pre 2011 values (but be totally willing to publish post 2011 values)
Also the probabilty that someone would actually think Vino is clean D:
Add all those things together - and to believe Froome you HAVE to believe all those things, and the probabilty is the above.
Contador - 0%. The chances that someone would have plastercizers in their blood but be clean. Im not even going to bother with Puerto or clen or his VAM on Verbier.