Resident Evil is kind of like Zelda to me. It's a franchise that's predictable by design, but throws in variations with settings, characters and gameplay gimmicks, resulting in an experience that's almost akin to an amusement park ride if you play the games in each series in release order and quick succession you can see the gameplay and design evolve right before your eyes in a rare sort of gameplay continuity. You can clearly see ideas from older games refined into staples, enemies with such a historical gaming scene relevance that you become like a murderous biologist guessing weaknesses and attacks from foes as soon as you see them because "Oh, I've spent five games killing squirmy things with fire, bet I should use fire on this squirmy thing," and that's a convoluted explanation, but one of you will understand the vibe.
Another point that makes them alike for me, and perhaps more relevant to this thread, is the fact that, according to my tastes, I don't really think there's a single bad entry in the mainline titles. There were titles that are less great than other great titles, but the rubric is based on overall competent games. There are some "evolutionary dead ends," I guess, gameplay or story ideas that were launched but quickly aborted or the devs realized the winds of the current fad were changing (i.e. RE5 trying super fucking hard to be a dude bro game, Twilight Princess going the srsedgy route) but there hasn't been a game in either series I haven't seen the merit of it's existence.
Because you guys asked.