Resident Evil 4 was the first one I played.
I re-bought RE4 last May on Steam after beating the game years before and it's still one of the best survival horror games ever made.
I loved RE5 as well, it was more action oriented but it was still fun.
Never played RE6 but heard mixed things.
For a series I would consider one of my favorites it's surprising to me that I still haven't really given any of the originals a shot yet.