By this I mean who is the best non-pure-casting class for combating casters?
I would think it would be some variant on a Mystic Ranger of the Arcane Order w/ Favored Enemy: Arcanist and the Wise to their Ways feat.
Wise to their Ways, for those who don't know, is from Ghostwalk. It takes your Favored Enemy bonus and allows you to apply it to saves against supernatural and spell-like abilities from your Favored Enemy.
That's a +10 to all your saves at level 20. Without any amount of real optimization.
So, two questions; the first is, what is the highest Favored Enemy bonus you can get by level 20? And the second is, what's your favorite anticaster?