Reaper is really easy for me since his head is really clearly visible and unobstructed during his animations.
Hanzo, on the other hand, is nearly impossible for me. I don’t know if they ever fixed that quirk where his arm covers his head, but his head is so janky.
Orisa’s head is the size of three Hammonds without their Ball. If you miss her head… ,wait, you can’t miss it!
It’s so big, even Yo Mama can hit it, and I’ve been informed that she’s so dumb she threw a rock at the ground and missed!
She’s also so fat, she got booped into Ilios Well and got stuck.
Orisa’s head is the size of the spawn ship on control maps 
Sym is extremely easy to heashot.
Torb and Orisa are up there too.
The most annoying to headshot IMO is Ana, at least from behind or the sides.
Easiest is D.Va
Hardest Tracer
and sometimes, but only on rare occasions - Moira if you have one of those A+D crouch spamming Moira’s 1v1 u, then her too, LOL but normal moiras ez to hit lol
Probably Torbjorn. It isn’t like back in the day where you could hit him basically anywhere and get a headshot. But it is still very nearly center mass. Rofl.
The hardest is Ana, no question, not even close.
Yo mama’s headbox is so big, Moira can hit it!
Easiest - Widow
Hardest - Idk i think my least played is Doomfist/Baptist.
I don’t know why my career profile says only 55 hours with Widow. Overall on console I’m around 103 hours with her. I play PC and Console. I even have the Season 6 Icon from 2017 i believe lol
Beardshot, maybe?