This is, of course, subjective, so go with whatever your feelings are. Also, you can choose to judge quality as based on the time of the game's release, or by current standards. However you choose to evaluate the question, I'm interested in hearing about which series haven't faltered in releasing good titles across a large number of entries or span of time.

I also think it would be fun to examine some that barely miss the mark: series that are almost entirely great -- if only weren't for those one or two titles that really brought them down.