Well we already know Reed was affiliated with them, so that doesn't really count, neither does Admiral Ross I'd say.
He's my favourite captain, and I love him to pieces, but it'd have to say Sisko.
The very episode after Section 31's introduction, he carries out a plan that is every inch their style. Hell, considering how low key most of their operations are I'd say the plan was far more brilliant than anything 31 could have come up with.
As for others, Riker's not the type, sure he's a Maverick but he has an incredible black and white sense of right and wrong, which he probably inherited from Picard, having served under him for so long. And as for Tom Riker, he didn't join Section 31, he joined the Maquis, a completely different organisation with a vastly different goal. And as the Defiant episode proved, he may have wanted to take up the terrorist's cause, but he did not have the stomach for their tactics or bending of morality. Both Rikers are very moral and righteous guys, no way would they join 31.
As for other picks, I'd say Janeway (she's just crazy enough), Eddington (pre-Maquis), T'Pol, and maybe, maybe, just if the situation was incredibly dire, Worf.