There's a few floating things, like the Broadcasting Centre, and the place the pegasus lives in...dunno if they float higher than Mount Avechna. I would think not, as the mount is supposed to be really, really, really high.
You can get higher than avechna's peak with the snow caverns. You can scale higher up, and fly at that. I don't know about Hallifax though, it could give it a run for it's money. Elostian's realm is high up as well, not sure.
For the purpose of the question, what would be the highest and the second highest if Avechna takes first?
Well, uh, how are you defining height, exactly? We're working off lore descriptions, not mechanical z-levels or anything like that. 'scale up' will never get you higher than Avechna, because Avechna is the highest point on the highest mountain, as per lore descriptions. The Book of Mugowumpois describes the House of Xyl as "Atop the highest--and coincidentally the most power-laden--mountain upon the First World". I suppose theoretically something might float up higher, but I don't think anything we currently have is that high up. Would need someone to check descriptions in Hallifax, I suppose?
As noted in the "lowest point" thread, most vertical exits don't necessarily mean greatest height, for example Celest's spiral road proves that you can move vertically without moving up or down.
Although I just thought ... where do the Havens count as being?
I think the Lawgiver spire might be the highest non-natural point in the basin. I'm not quite sure it's higher than the peak, but it seems the most prudent guess to me. TBC, The cloud gardens and the golden isle may be floating about as well, but I'm decently sure they're all below or in the clouds.
Everyone is a genius. But if you judge a fish on its ability to climb a tree, it will live its whole life believing that it is stupid.
Natural structure? Who knows! Existing? Probably the city of Climanti. I feel like Zoaka was the second highest mountain during the Elder Wars, but I may misremember. I also feel like it took some serious damage, exploding, etc at some point.
Not sure that you could actually work out the second highest. We don't really have a base to work from, like sea level in the real world. We don't know how high the bottom of the mountains in Icewynd are, so we don't know if Climenti would actually be higher than Zoaka. And then there's places like Dio and the new Lirangsha (I can't remember how it's spelt), and I don't think we have any idea how high those are compared to other stuff.
Answers
=P~