The Sega Dreamcast
Hmm... that's a really hard one. I'm assuming series is meaning overall, so I'd have to say Jet Set Radio. More than any other series out there, I would absolutely lose it if another Jet Set Radio was announced.
This is essentially my 'desert island' game. Grim Fandango. The story, the characters, the charm, nothing else has come close.
Duke Nukem. He's the king, baby!