I liked the original RE (originally played Director's Cut), but I loved RE2 the most (originally played the PS1 version). The N64 version of RE2 was the best though (and the best RE1 version supposedly was the exclusive GameCube one but I never got to play that one, but it's supposed to be great). There's a PC version for both RE1 and RE2 (RE2 Platinum) but I've never been able to find a retail boxed version (even used) copy of either of them. Although I'm not sure if they were ever released as a retail boxed copy (only images I can find of them being "retail" comes in a simple PC-CD plastic case without an actual box). I've never played the Dreamcast version of RE2 but it was basically the PC's "Platinum" version (and supposedly has soundtrack-related issues like the PC version has, such as simply missing or skipping tracks in some rooms, although that might be related to the CD itself not spinning in the drive or something along the lines, at least for the PC version).
If RE1 and/or RE2 were to be released on Steam or GoG.com I'd definitely pick up RE2 (if available separately). I liked RE3, it was decent and Nemesis gave me the creeps each time I heard or saw him. I also liked the randomized enemy placement (and items too), along with the custom ammunition/rounds system. But it wasn't as "atmospheric" and immersive as RE2 was for me anyway (or even RE1), but quite solid nonetheless (there's a PC version too, and the Dreamcast version is supposed to be relatively enhanced over the PC version, but never got to play the DC version). I never owned but rented Code: Veronica (Dreamcast) I think two times, never had time to complete it though (not sure why by now, maybe I just couldn't get into it for some reasons, since I used to complete almost all the games I rented), but from what I can remember it was fun. I do recall that Code: Veronica played very similarly to RE3 overall of course with 3D environments being the major difference, but game-play wise it was very similar to RE3. I never played other versions or Code: Veronica but I do know that there's an enhanced version (Code: Veronica X) for the XBOX and PS2 (not sure if it's "much better" or not).
I "skipped" RE4, saw a lot of footage of the GameCube version on YouTube a while back, gave me a good idea of what it was like, seems decent, probably entertaining enough like RE3 and Veronica were but maybe not as good as RE1 or 2. I know there's an "HD" enhanced version of RE4 on the 360 and PS3 but I've never played that either. I'm aware that RE4 received high praise but I also know that it was the moment the series went from "mostly horror" to "mostly action", so I'm kind of glad to have skipped it. I never played RE5 at all, skipped that one completely, barely saw any footage of it and I never bothered watching any video reviews... I don't even know what RE5 is about.
The last RE I played was the XBOX 360 version of Operation Raccoon City, a game that my cousin ended up renting. I came to his place that day, he had just started playing it for about an hour, showed it to me. I pretty much face-palmed at the horrendous "game" I was seeing before my eyes. It made no sense, wasn't about horror nor survival anymore (not surprising). It was all about "action" (even that part seemed awkward with stupid A.I., all humans and not a single zombie). He got to a point I think in the first "level" where there's some sort of a very poor L4D-style Tank'ish wannabe "boss" mutant redneck guy thingy... that desperately tried to be intimidating but only made me and my cousin laugh. I'm not sure if we were both depressed at that point, or if it was laughter because it was actually funny more than anything else.
Suffice it to say that right after that "boss" fight my cousin turned off his 360, took out the disc, placed it back in the box and we drove back to the video store for a replacement rental. The clerk asked "why" my cousin had returned it only like two hours after he rented it. To put it bluntly he simply replied "'cause this game is shit". The clerk hesitantly smiled and just said "...oh, well ok you can replace it". I spent the next half hour with him discussing how indeed shitty the game we just witnessed was. My cousin happens to love horror-style games (and movies), he loved RE2 (and adores RE3, I don't like RE3 as much as he does though) but he vowed not to approach another "Resident Evil" game ever again. I'm not sure if he ever played Code: Veronica or RE4 though, but I might discuss those two next time I see him.
For me that Raccoon City game also "killed" it, there seems to be no going back to the horror/survival themes, atmospheres, play styles and game-play mechanics (such as fixed environments, moving from rooms to rooms) of the earlier releases (RE1, 2, 3 and Veronica). Side note, I also know there's RE Revelations but I never played that one, although I'm aware it has generally bad reviews out there so I won't bother with that one either. I've also seen some of the reviews for RE6, it's supposedly the "best of the new ones" since a couple of years but reviews are very mixed for it from what I've seen, seems to be either "you love it or hate it" type of game. Maybe one day I'll try that one but I don't feel it's urgent to do so anytime soon.
My best memories of the franchise remain with RE1 and RE2 (mostly RE2), but credits where due RE3 had its moments too.