Last night I was working on a review of Splinter Cell: Chaos Theory, and the more I thought about it, the more I was impressed with, not only its quality, but the entire Splinter Cell series. I think it is slightly rare because each game in the series is not only good, but each one truly improves on the formula.
Then I looked in my collection at my PS1 Tomb Raider games, and remembered how they kinda got flat after the second one.
So this brings me to my question. Which series do you think not only started great, but has gotten consistently better with each installment?
I really like Metroid Prime 1 & 2, but I'm not sure how solid I think some of the previous Metroid games are. I guess I nominate the Splinter Cell series.
EDIT: I think to be considered a series, it needs to have at least three games. So sorry, "Halo" doesn't count.