For me it has got to be the characters and the boss fights. Since I could choose one I chose boss fights. I just loved everything from facing the tyrant to the grand fights with nemesis to the epic finish with wesker. It was all just great.
Not changing this sig until Final Fantasy Versus XIII comes out. Official Atlas of the god of war ascension board.
A long time ago on Christmas Eve, my deadbeat father (when he wasn't so deadbeat) gave me a copy of RE2 dualshock edition, after I had read a walkthrough for it in gameinformer back in the 90's
Back then before flatscreens and decent TV, I couldn't adjust the brightness on my television, giving RE2 a permanent dark tone, almost to where I couldn't see where I was going. That made my whole damn experience right there
It was the first video game where I had to man up and face scary moments by myself and solve puzzles... my first "grown-up" video game. RE has had a special place in my heart since then
What doesn't kill you, will probably leave a nasty scar
I'm somewhat surprised that the characters are the main reason so far. I'd be a liar if I said I didn't have an attachment to some of them though.
For me though, it's the classics. Blah blah alone in the dark did it first blah blah blah modernism. The traditional REs had everything I loved about Super Metroid actually. SM was one of my favorite games growing up, and I loved the tight corridors, the labyrinth style environments, scaling in power by searching for more powerful weapons and tools, the eerie feeling you get as you know something big is coming up, the immersive soundtrack, and how epic the journey felt the deeper you got into it. But RE did it in its own special way with more emphasis on fear and realistic "what if this really did happen" background that the plot allowed. The first time I played the original RE I was instantly hooked and to this day it remains in my top 3 game series whereas my interest in other series has waned over time or vanished altogether.
Definitely the classic gameplay/suspense/horror. Easily wasn't the characters. RE1 to RE2 had a completely new cast of characters yet the nothing was lost, only enhanced. Then yes Jill returned for 3, but that's it. Survivor, despite being bad, had new characters. So it was the gameplay that kept me in.