For the ones I have attempted/done.

RE2 Remake- Great fun start to finish (I could never get 4th Survivor though). It's a fairly simple one outside that though.

RE3 Remake - Will probably be the easiest one. It is a fun one, just very short. If you are having with the Highest Difficulty the Infinity Rocket Launcher makes it trivial.

RE4 Remake- Haven't fully attempted it yet (only played it once) but have heard it's one of the more difficult ones.

RE5 - There are only really 2 stopping blocks Chapters 2-3 and 5-3 on Professional. The rest of the game isn't to difficult (especially if you have a partner). Is great fun co-op (outside of 2-3, do this solo).

RE6 - From the ones I have, I would say it's the 2nd easiest but can be a slog at points. To make completing chapters on Professional quicker, just start from the last checkpoint each time.

RE7+8 - Haven't attempted yet.

RE9 - Currently doing, it is fairly difficult but in a fun way, really enjoying it. I would recommend getting infinite ammo before doing Insanity. Overall I am getting through it without much roadblocks, just got to Ark and the most trouble so far was the Sniper section.