Here's my opinion on all of them.
Resident evil zero: Had some great ideas, and puzzles, but wasn't that great.
Resident evil: The original game is far too clunky to be enjoyable today, however the remake is my favourite of the whole series, and guess what...it manages to be scary as shit, a first for the franchise.
Resident evil 2: Many people will tell you that this is the best in the series, i tend to always disagree. It had all of the text book Resident evil atmosphere we all know and love, but I didn't find it as fun as remake or Resident evil 3.
Resident evil 3: Perhaps in retrospect it was too similar to two, but with a huge zombie beast chasing after you made for some of my favourite moments in my video gaming memory.
Resident evil 4: Even though they changed alot in this game, it managed (for the most part) to be the a great original take on a series that was growing abit stale.
Resident evil 5: kind of a mixed bag for me. On the other hand they kept the resident evil 4 style of gameplay, and on the other, it felt too rushed, too short, and totally unnecessary.
Resident evil code veronica: The Characters were terrible (Alfred and Steve anyone?), but other than that it is a very fun, if hard Resident evil.
Resident evil Outbreak: (see Resident evil zero)