I can't say from personal experience, but from what I have heard, the Osceola is pretty limited in range, so that might be the toughest to get.

I'd say the hardest to get would be the ocellated.  They'll cost you a few thousand dollars and a plane ride to Guatemala which makes it pretty tough lol.  As far as calling I've heard that merriams respond the best to the call out of all the subspecies although the eastern isn't too bad either if you can get on the birds.  I would say the osceola and rio grande would be the two toughest from a calling perspective.

The tough thing about the Merriam's is the fact that they are more nomadic than the other subspecies. You will typically find easterns, rio grandes etc in the same creek bottoms year after year. Merriam's move around a whole lot more. Unless they are being fed which is a problem in a lot of places here. There is nothing more annoying than finding that the birds are holed up on some jerks land because he is feeding them. I have seen anti-hunters do this to keep "their" birds from being harvested and I have also seen guys to this so they can hunt them for themselves. It really gets on my nerves. I had to backpack in for turkeys last year because of this.

Just learned a lot about turkeys that i didn't know.  I just hunt the eastern turkey.