I don't believe you can fit a flat mount caliper on a post mount fork. Without some very clever engineering I think it would involve negative spacers. However, you can buy OTP adaptors to allow post mount calipers to be mounted on a flat mount frame. So I'd say flat mount is easiest to adapt.
If I have a frame that has Post mount front and Flat mount rear, what's the easiest set of calipers to adapt to fit, post or flat?