I've enjoyed all of the games as well so it's kinda hard to pick what I enjoyed most, but maybe possibly RE2R and RE4R (never played og4). 5 was more fun back on the 360 doing coop than doing the playthrough on Steam Solo, the Uroboros flamethrower fight is a buggy mess on PC so I said screw it and bought a rocket launcher lol.
It's a tough call but I probably liked RE Rev 1 more than RE Rev 2, even though my RE Rev 1 playthrough was NG+ Infernal since I had played it before so there were some moments that were absolutely soul crushing to get through lol.
If I ever get around to playing one of them again it would probably be RE2R first
For Resident Evil 6 it was an okay game, I absolutely hated all the superman jumping zombies on Leon's campaign though, and the Chris End boss fight was pissing me off, I was playing it on veteran, had practically no ammo, 1 maybe 2 healing items, and Piers was virtually worthless, The majority of the time he would either get knocked down and refuse to get back up (I could not help him up, he would just sit there), his lightning attack would either do no damage to the cocoon, or he would shoot the ground for some dumb reason. It was a lot of running around collecting the scraps of handgun ammo it would occasionally give me from a bomb, and praying that Piers would actually land his shot and help me break the cocoon open so I could stab the boss. I'm not sure how likely I am to give RE6 another playthrough anytime soon lol