When did they ruin it?
Easy: Code Veronica. Brought Wesker back from the dead (turning him into a Mary Sue. Then killed him off stupidly in a volcano because they couldn't do shit with him after Mikami was leading him up to being a big-bad super-villain with a bunch of viruses), terrible antagonist (Ashfords), just unfun campaign to go through.
0 has faults but it would've been superficial if it didn't have that stupid item gimmick to force you to use both characters.
Gun Survivor 1/"Survivor" was alright, but the lack of Guncon support in America was

Outbreak is about the only "old-school" post 3/Last Escape of the series worth the time.
4 was alright, but not a Resident Evil series entry.
5 was okay, but continuing on the downward spiral of turning the series into something it isn't.
6 was hot flaming

and there is nothing redeeming for it in regards to the series. The only GOOD thing I can say about 6 is that Mercenaries was alright with the "new system" they introduced. But everything about 6 was terrible otherwise and the series REALLY didn't need a third-person-shooter make-over.
It's telling when a re-release of a remake of the original sells faster than most anything Capcom has put out digitally. Which includes 4/5/6 on Steam and elsewhere.