Depends on the general, the cash value of the prisoners and the wealth of the nation of the prisoners. If the general is dread, I always execute. If he's chiv either ransom or release. Ransom if I think they can pay or I don't want them to live but general has good chiv.

I once made the mistake of releasing a bunch Scots instead of ransoming them from an army lead by King Rufus the Chivalrous. I was trying to bump up his chiv even more. Turns out I only killed 1/8th the full stack army and routed the rest, but I suffered enough casualties to leave approx 2/3rds a stack (was not a full stack to begin with tho').

The stack came flying out of Dublin next turn and obliterated the entire army. If I'd executed them, Scotland would be finished. Dublin would have fell easily as the garrison was puny. Moral? Kindness doesn't pay! Everyone gets burned/executed now. None of this be nice to your enemies. Attack me, you die! Defend yourselves, you die!

Well not really, I'm just very bitter about that. A very costly mistake to make.