Well, it's a bit tough to seperate Mjolnir from Thor, so they're part of the same package.
If he drops Mjolnir, it just flies back to him, sometimes smashing his opponent in the back on the way (sneaky hammer!
)
I did try to work out how Thor would fare without Mjolnir, here's an idea:
He has extreme strength and resistance to damage.
His strength level can increase when he's enraged.
Thor wears armor made of Asgardian steel and a belt that augments his natural strength.
He can also fire an extremely powerful energy blast from his hands that has slain enemies who were otherwise unkillable, although this leaves him in a weakened state.
Likewise, he can fire lightning bolts from his hands.
He may also be able to control the weather to some degree.
It's important to note that although he uses Mjolnir to focus his powers, he doesn't actually need to use the hammer to do so.
Those last ones may be problematic for Superman to deal with, especially the energy blast one.
And since they're magical powers, he'd be vulnerable to them.
It's hard to come up with a proper baseline, since it seems in most cases, Superman's powers are exaggerated for dramatic effect.
But if we look past all the hyperbole, it seems even without Mjolnir, Thor is an even match for Superman.
So, if we factor in his magic powers, I'd still say Thor comes out on top.
As for Kajiu, unfortunately, that would be a fairly short battle for pretty much any superhuman to win.
They're strong, but not well armored.
The strongest heroes would end up gutting them like a fish.
I've now got a mental image of Colossus standing on a chest of a kaiju, opening up it's ribcage like peeling an fruit and pulling the heart out to hold over his head.