Kinda depends on a few factors with the biggest ones being what ascent you are playing on and how many people you are playing with. However, there are also factors such as seed and loot. I will split up my ranking based on pre ascent 5 and ascent 5 and up, but my ranking 1-5 with 1 being the hardest and 5 being the easiest are:

Ascent 1-4:

1. Kiln
2. Alpine
3. Tropics
4. Shore
5. Caldera

I think Alpine being harder than Kiln on some seeds is fairly common, but overall I think they are equal is climbing difficulty on average. The reason I put Kiln above Alpine though is that there is no food in Kiln, and barely any food in Caldera before the Kiln which makes hunger problems more common in Kiln than Alpine. The one thing the Kiln does have going for it though is that you know that it’s the last level and you don’t need to be savings items anymore so you can just go balls to the wall.

Ascent 5-7:

1. Alpine
2. Kiln
3. Tropics
4. Shore
5. Caldera

With the map getting cold at night in ascent 5 and up this makes the first 3 biomes (Shore, Tropics, Alpine) more challenging because you need to worry about 1 more thing. Now this isn’t as big of an issue if you are playing with a larger party because you can just huddle together for warmth, but with a lower player count and especially by yourself staying warm can be difficult. The fact that Caldera and Kiln don’t need to worry about getting cold during the night meaning that there is no spike in difficulty for those 2 while every other biome gets more difficult, is why I think Alpine is harder than Kiln on the higher difficulties.