Amid Evil

Arcane Expanses is one of the most interesting episodes, with some truly lovecraftian architecture on one hand, and some of the longest and hardest levels on the other. The most interesting parts are the huge water bubbles hanging in the air, than you need to traverse. Only natural that the fight with the twin bosses of this episode occurs in one such bubble.
In general, the game is very good at not abusing mechanics. For example in the Arcane Expanses you meet snakes spitting acid. Stepping into an acid puddle damages you slightly. This is the only place in the game you will meet those.
The Forge episode with levels such as “Gauntlet of Torment” is probably the least favorite of mine. The only positive thing I can say about this is that the boss looks inventive.

Solar Solcitce is probably the most tactical episode. I also like how the Sun sets once you kill the Solar Saint.

The final episode is hard, but interesting. The geometry is against you, you fight on tilted platforms hanging above the abyss.
The final boss looks like a mushroom trip, but a short one, and in a good way.