That is a real difficult question to answer....usually the hardest egg to hatch is the one you want the very most in the world TO hatch.
Philosophical answers aside, peafowl can be danged hard. I have also heard ostrich and emu can be hard since if there is too MUCH humidity they die....
Someone once mentioned that Seramas are very difficult but I (only based on personal experience) have found that those through the mail didn't work for me.