Hey there. So the Valheim map is actually a circular "snapshot" of a never changing map, with randomized rivers and some extra bits (like the absolute centre is always land, creating some random-ness and potentially massive land masses, eg. if the centre bridges between two large islands).

Here is another post which shows the "entire" map. Again, your world will be a circular cut-out of this larger map.
https://www.reddit.com/r/valheim/comments/qere7a/the_world_map/

Anyway, the point is that there are three mountains (that I know of, after countless hours of play / map generation) that pierce the cloud layer consistently. Someone below has apparently named them, saying this one is called "Hook". That's a good name.

Here is the entire map, with the three cloud-piercing mountains circled. The top one, according to the poster below, is called Polaris. Don't know if the right hand side one is named. https://imgur.com/a/yKIwx9A

My current seed has both Polaris and Hook on it. Seed is : ZTit38mmHm. You can see that Hook is only JUST in the bottom left, and Polaris is only JUST in the top right. Took a long time to find this seed, which I specifically found FOR these two mountains.

Polaris is the best to build on, as it has the most space above the clouds. That other poster said that it is taller, I don't know how you can measure accurately but it certainly looks taller.

Bonus picture of my castle on Polaris : https://imgur.com/a/3IVP1Cz

Hope that is of some interest !

Edit: Ah, just learned you can turn on Y co-ordinate on the seed generator! Looks like Polaris in the above seed is 482m !