Man, it was funny when one or two people say "link and loop, that is great...blahblahblah", but then a bunch of unoriginals followed and it is pretty dumb now.
Anyway, to answer the question:
A) Backflips are easiest in terms of the pure science of it. Your body and momentum all already headed that direction. Watch anyone go off a jump who doesn't know how to pop. But, they are by far the most mentally difficult of the three because of the consequences of not commiting fully.
B) Front flips are in the middle, in my opinion. They aren't easy or hard both physically and mentally. You can't really see your landing well, which woul dbe the only sketch part, but if you are doing them into powder then coming up a little short or over rotating a little aren't too bad. But still, your momentum is not with you.
C) Lincolns are in my opinion the easiest and what I learned first. You an always see your landing and it is pretty easy to get the momentum to do it. Body control is easier because you can always spot the landing.