It kind of varies depending on the level in question, and also whether you're online or offline, but generally I'd probably rank them like so.
From a gameplay/balance standpoint and assuming high skill play
S: Kevin, David, Jim
Ridiculous damage output, though David takes a lot of skill to use well and Jim needs time to set up, Kevin's gun with his potshot is ridiculously broken to the point bosses are complete jokes if you save all the ammo. Kevin and David both have a good running speed too, and while Jim doesn't, his dodge is so strong he'll basically never get hit unless mistakes are made.
A: George, Cindy
Usefulness is dictated by how much your team gets hit, but both of them have very good personal skills with their dodges so they are good for running objectives, and George gets significantly better in file 2 where he can become a strong boss killer by sacrificing un-needed herbs. These two also work very well together, Cindy letting George have a headstart on pills or antiviruses. Also shoutout to Cindy for being the only character who can carry Linda and avoid the sniper shooter in End of the Road at the same time.
Another funny tidbit: George's pill gun does DOUBLE healing on nightmare mode. If it's nightmare mode, I'm almost tempted to put him in S tier.
Kind of gets the short end of the stick. He is solid for his HP stat and block, and the super swing helps sometimes, but it's offset by how slow he runs and him having a bigger hitbox, and he has nothing unique in any level unless you count Bob in Outbreak, which really just makes the level harder more than anything (even if it is fun to save him). Playing as him you just can't avoid getting hit sometimes, so it's a good thing he can take a beating.
Just doesn't really bring anything to the table. If you use her Yoko: Z variant she can make a good AI partner offline to carry key items for you, since that variant has double the HP, but base Yoko will die too quick as an AI and actually playing as her is equivalent to a challenge run (UNLESS it's Below Freezing Point where she has a level benefit). Only good thing about her is her incredibly slow virus gauge, making it not matter if you get downed a few times. Her dodge-crawl is fun to use though but from a viability standpoint, there's just no reason to pick her besides having fun.
I can't rank Alyssa because she's entirely dependent on the level. She can be basically mandatory (Like in Wild Things where there is *8* locked doors for her to pick on VH), or just a much weaker version of Kevin (Flashback, Below Freezing Point, at best you get an extra first aid spray here).
Now, for my personal favorites based on aesthetic and personality.
Cindy: Probably my favorite video game character ever. Poor, pure girl who doesn't deserve to be in this situation. Love her a lot and I'm usually playing as her online 95% of the time, helps I enjoy playing healer for my friends.
Yoko: I find her incredibly interesting, especially her ties to T and Umbrella. Kudos to her for bringing about Umbrella's end.
George: Just a lovely man. Hardworking and stubborn, someone you wouldn't look twice at if you saw him IRL, he looks amazingly normal. It's my headcanon he gets together with Cindy after the incident and takes care of her.
Mark: Best dad. We could all use a dad like Mark. A+ voice-acting.
David: A character who could have really used some more development, I would really love to know why this plumber comes across as a gangster. Way too brave for his supposed occupation!
Alyssa: Very angry, her flashback story is incredibly interesting to me, enjoy her personality the most. Apparently has the biggest balls on the cast.
Kevin: A lesser favorite, but I enjoy his backstory and relations with people in the RPD. It's cool how he is an even better shot than Chris or Jill, and the only reason he's not STARS is because of his anger problems and drinking habits causing him to get rejected. Probably a good thing he failed the application though, or Nemesis might have shown up in one of the games.
Jim: Used to hate him, and while he's still my least favorite, I get a lot of laughs at his antics and voice lines now. Don't know why he squeaks in file 2, scared to ask.