It's a tough question for me because I love the series. Symphony of the Night is likely the winner because I've played it more than the others but Castlevania III is superb and the GBA entries are charming as hell, especially Soma Cruz entries. I still have a handful of entries I haven't played yet, but it's hard for me to think of a truly bad entry in the series outside of the Gameboy entries.

This collection is pretty awesome it seems, Konami has done a good job with them as the classicvania one was superb as well. I imagine they'll do a DS collection next, and possibly a PS2 one as well. Sadly, I still wish Konami had released Castlevania Legends since it's the only handheld entry that hasn't had a re-release outside of the DS ones.