I'd like to cheat and say it's a 3-way tie between Claire, Leon, and Ada. But if I had to really choose, I'd say Ada.
It's funny, b/c I actually thought of this just yesterday and how our age when we first experience characters skews our perception of them.
i.e., I was a kid/young teen when I first played RE1 and RE2. Chris and Jill were older and more competent, so I related to them less. Leon and Claire were younger and inexperienced, so I related to them more, and therefore liked them more. ~25 years later, I now wonder if I'd like Chris and Jill more if I were to experience the series for the first time now. But Leon and Claire are portrayed pretty well in RE2make, and even though I've outgrown the "young, dumb, and naive" archetype of characters (especially in Japanese media), my experience w/ Leon and Claire from OG RE2 and CV cemented them as my faves. And yes, I'm aware they've matured as the series progressed.
...but anyway, Ada because best waifu. And regardless of my age, I've always had a thing for older, dominant women.
Albert Wesker followed by Leon S Kennedy. I find it amusing you can punch, shoot or let a Tyrant impale Wesker and he gets up and walks away like it was nothing,lol I did not like how the movies potrayed him as I do not remember him mutating his mouth in any of the games, He mutates half of the one side of his body in RE5 but not his mouth. Leon earned a spot on my favorites list the first time I played RE2, but I love him in RE4 as you never know what he's going to say,lol "Your right hand comes off?" "Where did everybody go? Bingo?" lol I may have the quotes a bit wrong as it's been awhile since I've played those games, I just started playing RE0 again...yeah you learn a bit about Wesker in that game as well and some of the stuff I've seen as I read the files you find over a few times and one of things makes me wish they'd do a backstory game/proper movie on Wesker...as Albert Wesker what in the world were you thinking? One of the games on the Wii you can play as Wesker which I like to do, need to find the zapper/gun thing for the Wii to make the game easier. RE4 has Mercenaries Mode which took me awhile to unlock, I use Wesker in that one as well, he's mine no one else can use him,lol Also I write fanfics and do crossovers between series...one of them being a crossover between Resident Evil and Monster High...Wesker has a daughter named Sophie in that story and she's under the care of Jake, don't want to spoil RE6 for those who haven't played it or got to Jake's part, but he's an awesome character as well, so yeah Albert Wesker inspired me to make an O.C that is relation to him and use her for a fanfic,lol Sophia Wesker is her name but everyone calls her Sophie.
Probably Barry and Jill, if I'm being honest with myself. It was Jill's campaign that really hooked me on the original RE. I loved Chris' as well, but I liked the way Jill and Barry played, and I liked their side of the story more. I'm a sucker for a good betrayal-for-the-right-reasons.