Definitely Nebula by Far.
Vortex: The easiest. None of those guys can hit through walls.
Stardust: Second easiest, just wall yourself in to protect yourself from Flow Invaders, then Solar Wrath all the cells/weavers/everything else.
Solar: Not too hard at first glance, I stand on top of an elevated platform where Crawltipedes won't chase me (since I'm standing) , Selenians and Srollers can't reach me. But those goddamned Corites and their wallhacking ability...
Nebula: FULL of irritating ****. Brain Sucklers are fast, appear in numbers, move through walls, and facehug you to make vision minimum, Nebula Floaters fly and teleport around the place (I use Solar Wrath so I can hit these two through the ground), Predictors are just annoying, and it's impossible to stay in one spot thanks to the indestructible wallhacking magic balls that those Evolution Beasts spit out.