You can only go on current form really (i.e the last season) and so I'd say Distin, by a decent margin.
Jags had some brilliant spells but dipped in form, though injuries obviously impacted that a lot. Stones was immense but still only played half of Distin's games. Alcaraz is clearly the best centre back in the world, but he holds back his ability to make the rest of them feel better.
But seriously, Distin was a man mountain last year. Jet heeled, the physique of a tank, helped Stones out no end as our only seniour CB for long stretches. He's a hunk.