It's subjective per person and changes per fight. Perceived difficulty also changes dramatically when entering savage/ultimate and even more when optimizing. I'll only really touch on the 4 DPS I actually play frequently.

SAM is insanely easy. Smooth brain extreme. Can do disgusting damage even if you ignore positionals. RPR is also monkey brain when it comes to full uptime, but does suffer from the same issues as MCH with downtime fights. Now without Kaiten, SAM is definitely easier than RPR. It has tools to recover from minor mistakes with minimal DPS loss where RPR, like MCH, is gauge based and minor mistakes become massive annoyances that you spend too much brainpower on to recover from.

MCH is easy to optimize in full-uptime fights, but suddenly it becomes one of the hardest jobs to optimize in fights with downtime (see DSR). It's a very rigid gauge job so one flubbed input and your entire rotation and gauge is thrown out the window. You broke a combo by accident? Congratulations for having fucked up Queen for the rest of the fight. For me, MCH was leagues and leagues harder in DSR than DNC.

DNC is very easy on the surface, but starts gaining a lot of depth the more you look into it. There are a lot of choices to be made on the fly while playing DNC and many, many opportunities for micro-adjustments.

Personally I think DNC blows SAM/RPR out of the water when it comes to optimization complexity. MCH is a per fight basis - can be fairly easy on something like P6S/P7S but becomes the absolute worst on fights like P8SP2 and DSR.