Well, I almost wanted to pick what i thought was an actual rivalry. But Chris & Wesker, Leon & Krauser, I thought Steve & Alferd was a nice choice, but i didnt choose them.

I dunno which would be the BEST in my opinion, but Chris and Wesker was obviously the most important, while Leon's and Krauser's was really good and the Jill v. Nemesis one was just awesome.

Chris Vs. Wesker

Also possibly the biggest battle of the Series