According to RE.net it's Rachel I use most. She's an expert with shotguns and magnums, her health regenerates, and I like her a lot for some reason.
I usually equip her with the Sledgehammer shotgun, the Government pistol and one of the magnums.
Jill specialises in handguns and machine guns, which happen to be my favourite weapon types, and as Mantis said, her knife animation is very fast and useful. I would love to dress her as the pirate princess, but Ghost Ship is a pain in the ass for me on my current level!
I'll stick with her BSAA uniform until I beat that stage.
Then there's Chris. I don't like his winter outfit and haven't unlocked the BSAA uniform yet, but hey, it's
Chris, so of course I use him a lot.
Especially now with his sailor costume, that's the best idea Capcom ever had, and it comes with a unique special ability too.