I personally think Skyblock is better because it can really show you what can come out of a bucket of lava, a block of ice, a tree, and a couple of dirt blocks. Four Pillars has a lot of the blocks from the game, so there is a little bit more flexibility with what you can do. Four Pillars was based off of Skyblock anyways XD.
I think skyblock is the best, because it's the hardest and have the least amount of resources. Also, it may be the first "Sky" survival map that started it all. I never played stratosphere survival, but It looks too easy, with so many resources and stuff. Four pillar is nice, it gives you more resources, but not too many resources. It almost tied to Skyblock.