That's kind of a tough call.
The only Castlevania games I really like are Super Castlevania IV, SotN and one or two of the portable games.
I really like the NES Contra games, but the rest of the series doesn't interest me.
Gradius I never paid much attention to. I like the Salamander games, and I know they share a lot in common, but there are so many damned space shooters that I just never gave a damn about Gradius.
I guess I'll just have to go with Castlevania.
I can at least say that I like every game in each of these 'series', and would love to see them both get updates/sequels of some kind.