Absolutely B with bases loaded. Where are the likely plays? Any ball hit on the infield the ball is either going home, then to first. Where does the BU want to be, where he has a good look at the banger at first. The other likely play is the closest base to the fielder and then first. Either way, you want to be toward first, so your starting point is B.
R1 only-B R2,R3-C
R1,R2-C R1,R3-B
R2 only-C R1,R2,R3-B
R3 only-usually C but many go to B with two outs.
I wouldn't say "absolutely." I have used C in a very particular situation with a specific team that loved their catcher's cannon. I got burned in B the first two games with this guy. I figured I'd go to C because I'm not a tree and I know how to use the working area, and I'd have more time for that than trying to get an angle from B on a throw behind R3.
And as long as we're on the subject, some collegiate guys up here (with the blessing of the conferences and national coordinators) have started experimenting with B at all times with runners on.