1st of all.. what are they called? It's driving me nuts.
Then what are your favorites? What do you still consider a great game now that the next game(s) in the series are out? Are there any you like better than the sequels? Try to filter through the nostalgia effects if they are series's you grew up on.
For me, Kingdom Hearts is what I'm talking about. Fantastic. The ONLY issue for me in the entire game is the camera control lol. Great original storyline. Fun gameplay. (Minus camera control that requires serious getting used to) Lots of replay value. Etc.