It's not the nut, so much as that stupid little plate under it.
When I replaced the sprocket on my bullet (due to wear, I ride a LOT, ok..) that gave me the most hassle, and I thought it looked rather insubstantial, so I made a new one out of heavier stock and replaced it...easy enough to do, a simple metal cutout, is all.
So don't let it break your heart if you mess it up, just have some sheet stock handy.
What works best to get that damn nut off, is a way humongo adjustible wrench and a rubber mallet...and don't take the pedal arm off first, cause that'll give you something to lean on/hold on to/use for leverage.
Apply that adjustible, and make ultra-sure it's as perfectly tight as you can get it, and use a RUBBER mallet, on the end, most of the way down, wham-wham-wham....like ole john henry bustin rock, and sooner or later it will come loose.
Once it's loose, THEN take off the pedal arm - unscrew the nut, and put a wooden dowel rod to the end of the retaining pin, and hit THAT with the rubber mallet too....ba-bam-ba-bam, and that'll come out...eventually.
Then yank the pedal arm, and unscrew the nut, and get a small flat-blade screwdriver....use that to pry that damn plate off, preferably without damaging it, go one side and then the other, and have patience.
Once you get alla that done, it's a snap to replace the sprocket and smack it all back together.