The multiple land or water retired guns. This is where the rubber meets the road, the true test of "MARKING". Training without the talent will only get you so far in most situations, true talent will shine like the North Star.
Most difficult for us or the dogs? A passed on dog of mine line manners or lack there of was her thing. Yet other dogs of mine at the same time had excellent line manners so apparently I was able to teach one and not the other. Another dog, another time it was multiple marks. Finally had to admit that not all Chesapeakes are able to count to three. Same dog never cheated anything, land or water. Never knew a pile of scrub or set back of water it couldn’t plow straight through. If difficult equals amount of time and effort required. I think now it is the blind retrieve. Obedience a close second as maintenance for both is always ongoing.
Other - it really depends on the dog & what they have for a hangup, some do some don't. If they have a hangup, that's going to be difficult . But there are many who would say my dogs could use a little more line manners. I prefer to do whatever it takes to leave the maximum natural talent in a dog's performance rather than having a dog that looks like a robot. Seen a few of those in my day.
This is a tough call! Depends on which dog I am talking about. We all know the black dog will never be 100% sane at the line, but can do "all that other stuff" if I will just sit in the truck and wait for him.
But now, I have another who finds it amusing to learn a new skill flawlessly, and then decide to play a different game than me when he gets to a trial!
If all I had to teach 'em was multiples, retired guns, blinds, etc......that would be EASY!