So, I've been wandering around a ridiculous number of really dumb places on the web today and found something so stupendously idiotic I just had to share it: In a full power battle who wins? [One Punch Man] vs [The God Emperor of Mankind] (40K)
Can't find the original page again: but here's what you need to know to0 play the game: https://otakustream.tv/anime/one-punch-man/ VS http://warhammer40k.wikia.com/wiki/Emperor_of_Mankind GO!
To me, it all comes down to one question: how well can OPM handle telekinesis? If he can shrug that off like he does most things, G-Emps is about to die. If not. G-Emps picks him up like a rag doll and slams him into the nearest convenient mountain repeatedly till he stops moving.
I'd have to give this one to the God Emperor if only because OPM has human weaknesses. Not being able to breathe underwater, or in space, for example. Or because Emprah would just hurl OPM into the Warp and spend forever punching demons into paste.
He got kicked to the moon once, then jumped back. I'm aware that he makes a motion to hold his breath, but it seemed more like an afterthought after realizing where he was and he's obviously neither affected by the extreme temperatures on the moon nor the vacuum of space.
However, I'm going to call this round in favor of G-Emps because of that other thing that was brought up: he'd just chuck OPM into a warp portal, like that one at the base of his throne. (And if this results in OPM doing a Round 1 TKO on the chaos gods, as far as G-Emps is concerned, so much the better...)