Game series that peaked with their first game vs. game series where the first game wasn't nearly as good as later ones

[Tron 2.0]

I was thinking of the first metal slug short and simple run'n'gun that holds up still but in some ways,it's sequels did out do it.Such as metal slug x and metal slug 3.Though after part three snk was never able to recapture the magic of the first three games in the series.