Battle, by 
It's tough to talk about the eyecandy in a level like this that uses a new, lovely tileset, because the temptation is there to blame it all on the tileset, rather than the level author. But we've all seen plenty of hideous levels using Nick's original sets, so it can't be that simple. So much of this level depends on how detailed the background is, filled with so many different layers with all different speeds. Numerous cave extensions use layer speeds that are not quite 1/1, artfully creating depth—this kind of layer use can be some of the most time-consuming but it can also look the best, helping the level to feel like a real place and not just a 2-dimensional cutout suspended in space. There's plenty of detail on the floors too, with animated grass and stacks of rocks, and the ceilings are all jagged in ways that don't interfere with gameplay. (On the other hand, I don't love the walls that appear jagged but are actually smooth due to invisible masks. And for that matter, I don't believe the level is really a mountain.) Everything feels deeply natural, in a detailed way… it's plausibly organic, not designed.
Unfortunately, the same can be said for the layout. I've been wandering the level for a while now, trying to review it, and nothing sticks out at me except for the giant empty space toward the left. Everything else feels like an inpainting algorithm. Lots of tunnels and thin wooden platforms everywhere, sometimes a powerup, and I'm just not convinced this all forms a cohesive whole. The thin vertical bits with the seeker ammo are kind of nice, though I don't even know that seekers are all that meaningful here. Making a Battle level is a different experience from a CTF level because there's no obvious structure you can depend on: you're not drawing paths between two bases on opposite ends of the level, you have to come up with your own flow. And I haven't been able to find the flow here.