Mephisto

Mephisto: A level II(?) demon and one of the Lords of Hell. His power levels vary but at his highest strength I'd say he can go toe to toe with (though not succeed against) omega level entities. He's one of many different devil villains populating the Marvel Universe but what makes him stand out is he may be possibly as annoying as if not more annoying than Loki.

Primarily he antagonizes Ghost Rider but he's also noteworthy for f*cking up the timeline continuity of reality in Spider-Man some time after the Marvel Civil War as well as tormenting the Silver Surfer once in awhile (in effing space. It turns out Mephisto doesn't really need to eat, breathe or even sleep so he's like "idgaf lololol." ). In between all of that he's also made time to be a dick to Thor. Basically, this villain really gets around. Like Galactus he too is responsible for creating a ridiculously high powered entity. Instead of endowing a herald with the power cosmic however, Mephisto is the guy who is the jerk that bonded Zarathos to Johny Blaze (I forget the specifics but Zarathos made Meph uneasy and the best way of dealing with him so that he wouldn't be competition toward Mephisto as ruler of the dimension they are from is by binding Zarathos to mortals).

So, obviously Franklin Richards is stronger than him but I'd say Mephisto is somewhere between The Beyonder and Franklin in terms of power level... It's sketchy though because like so many comic book heroes and villains his upper limits aren't clearly defined and once you think they are because it's got an entry in the official handbook then the next team of writers alter it again so that things change in the next edition.

Reference images:
[link]
[link] <- apologies for the crappy res that site chose to have their picture of the cover be. It doesn't do justice to how awesome it is the way the actual TPB looks when it's physically in front of you.
[link] <- Crystal of souls.