I would have to say that its a tie between the first and third games. They both held tight to the survivor horror aspect, but added great game play and inventory selection. Also the addition of Mercenary's gave RE3 a great add on to the already awesome replay value. (And the remake of RE1 on the game cube..F#@#ing brilliant, not to mention better voice overs. XD)
If you mean a brand new Resident Evil game, not for a very long time. They are talking about "Rebooting" the entire series with new characters and plots. But it is still in talks.
They might come out with older R.E games with new content, but a new game, no.