lets see....
well, problably...Corruption.
I agree, Corruption is gross (the enemy is a WORM GEEZ) but the boss itself...easier.
I mean, while Eater of Worlds can just travel in walls, he cant fly.
Brain of Chtulhu flies, second form, and MINIONS.
Eater of Worlds also have his "minions", when you kill one of his parts he turns into 2+
But still, the multiples Eater of Worlds are smaller and still cant fly.
And Brain of Chtulhu and his biome is even more gross (while corruption blocks being just a lot of virus and its mobs being virus in worms forms) since Crimson is a lot of BLOOD, MEAT and BONES in the background, while corruption is more natural virus.
Also, almost everything in corruption will be worms and same eater of worlds AI, soo if you beat eater of worlds the corruption becomes your own biome.
But crimson...spiders, ao onis face monsters, Chimeras...all with different AI.
And the boss fly already and his minions also fly.
Soo I prefer Corruption, because its easier to beat.