These rivals come without any sibling rivalry.
Since some of them work as tag teams,2 choices are offered.

I love when Thor has to mix it up with the less cosmic entities. So I went with The Wrecker. While I really like both the Absorbing Man and Skurge, the Wrecking Crew just strikes me as a fun mix.

And then there's the Enchantress, who is just such a great slinky villain. I like slinky villains.

I picked the Cobra and Mr. Hyde just because they were a rather odd mix to team up.
I enjoyed those early J.I.M. issues as a kid.

Other - The Hulk and Hyperion.

The Absorbing Man, just because he's become not only a foe of Thor but a Marvel comics mainstay ever since he first appeared. He's fought just about every major Marvel hero at some point. He has such a great gimmick, too, I had to vote for him.

His recent re-invention (in The Immortal Hulk) is another example of how versatile this character is.

Hercules, because Marvel Hercules is usually fun to read, and the Destroyer, because it has a cool design. Actually I first saw the Destroyer during its stint as a Herald of Galactus, so I tend to think of it more as a Fantastic Four foe.

Cobra and Hyde seem like they'd be seriously overmatched in a fight with Thor.

Surtur and Ymir have the advantage of being inspired by actual figures from Norse mythology, but are too huge and awkward to use very much.

The Enchantress? Well, in principal a strong villainess is a good (Good-bad?) thing. However I find the "slinky villainess" stereotype kind of grating.

I have vivid youthful memories of a story where the Grey Gargoyle somehow went from run-of-the-mill gimmicky Silver Age villain to captain of a fearsome pirate star ship. Talk about getting jumped up a level!

Say that four times fast

My fav Grey Gargoyle moment

NAKED STONE!

THINK OF THE CHILDREN!

Absorbing Man.

Absorbing Man and Hela.