I've played all of the main games in the series - 0, 1 (both original and REmake), 2, 3, Code Veronica, 4, 5 and 6. Haven't played Revelations or any of the spinoffs like the Chronicles games on Wii.
Still to this day probably one of the scariest games I've ever played thanks to its new enemies (particularly the crimson heads and Lisa Trevor). It also holds up a lot better visually than most games from that era thanks to the use of pre-rendered backgrounds overlaid with details done in real-time. The voice acting and soundtrack are a massive improvement over the original.
2. Resident Evil 4
Found the perfect balance between retaining the horror roots of the series and updating the gameplay to be faster-paced, building up into the most incredible crescendo of set-pieces in the final chapters of the game. The "save the president's daughter" storyline is deliciously camp and it's right up there next to REmake for scares thanks to creatures like the regenerators, chainsaw-wielders and 'it'. Also, the small section where you play as the near-defenseless Ashley? Fucking terrifying.
3. Resident Evil 0
The approach of controlling two characters is interesting and, although it does detract somewhat from the scare factor, it makes the game very unique and provides some of the best puzzle-solving in the series. It's also still the prettiest game in the series in my opinion - it holds up fairly well technically and has my favourite artistic direction of the series (the train that the characters occupy for the first couple of hours of the game is particularly stunning). Strongest writing in the series, too, with Billy being a bit of a mystery until about two-thirds of the way through the game. I also have to give it bonus points because Billy is hot. Like, really, really hot.
See a theme here? I think the three GameCube-era RE games were the best in the series. I'd actually love to see Capcom work more closely with Nintendo on the series again, because I think Nintendo brought out the best in their developers. It's gone so far downhill since then.
4. Resident Evil 2
Also has an interesting story and the really cool twist of playing as two characters and seeing the same events from different perspectives. It also has two particularly cool settings in the police station and the abandoned factory, and the "Nemesis" idea used in RE3 was basically started in this game with the creature that chases after Shelly.
5. Resident Evil 3
While the game's biggest idea (Nemesis) is basically just a more polished version of something RE2 did, this game is still a ton of fun. The ruined-city setting that Jill must traverse is probably my single favourite setting in the entire series, and the game is all-around enjoyable if perhaps a bit less distinctive than some of the other titles in the series.
6. Resident Evil 5
If this wasn't called Resident Evil, it would be great fun. As it stands, it's a perfectly enjoyable co-op shooter, but it's also the least scary game in the entire series and that counts against it. It simply doesn't feel like Resident Evil at all.
7. Resident Evil
It's aged pretty horribly and the voice acting is just so bad, but there's something enjoyable about the B movie-ness of the game. It's enjoyably trashy.
8. Resident Evil 6
There are parts of this game that are excellent - some of Shelly and Jake's storyline and most of Ada's storyline are genuinely good fun, and while they're not necessarily scary there are some great moments of tension. Unfortunately, so much of the greatness of the game is padded with even larger quantities of mediocrity, with an overuse of quick-time events, and with boss battles that just won't fucking end. If the Leon/Helena storyline and the atrociously bad Chris/Piers storyline had been scrapped entirely, and the Jake/Shelly and Ada ones had been rolled into a single story, this game might have been one of my favourites. As it stands, I still go back and play some of my favourite chapters on occasion but I doubt I'd ever play the game in its entirety again.
9. Resident Evil CODE: Veronica
At first I was going to say that perhaps I don't like this game because I played it much later and there's no nostalgia factor, but the same is true of 2 and 3 and I thoroughly enjoy them. There were just lots of niggling little things in this game that kept me from enjoying it, whether it was the awkwardly moving camera (unlike the static camera used in 0, 1, 2 and 3), the frequently bland-looking locations or the voice acting that's bad, but not bad enough to be entertaining like in RE1). Also, it's the only game in the series that has no difficulty options - there's only one setting, and that setting is pretty tough. For someone who likes to take their time with games, this really killed my enjoyment.
That said, I don't hate any of them - even 6 and CV had parts I liked a lot.