Although not a Skyfather, Surtur is one of the characters in Marvel Comics who is considered to be on that tier of power. Essentially, he is Odin’s equal and, as such, he is more powerful than Thanos.
As standard, Surtur wields the powerful sword known as Twilight – a weapon he forged in the centre of a galaxy (see below), resulting in the explosion of said galaxy, which Surtur was obviously capable of surviving.
The sword is capable of manipulating vast amounts energy – even greater than the energy manipulation feats of Thanos – and has been shown to shatter dimensional barriers and even inhibit Odin’s powers. Without it, Surtur is honestly around Thanos’ level. With it (which, as already mentioned, he always is), he dwarfs Thanos.
The fire demon from Muspelheim is also an extremely powerful magic user and can alter matter at a molecular level.
He is set to appear in Thor: Ragnarok as part of the Marvel Cinematic Universe and there are rumours he might be a servant of Thanos – something that would never happen in the comic books (unless Thanos was wielding the complete Infinity Gauntlet or something).