FFXIII-2 is the Magnum Opus of Final Fantasy. But there isn't really a Parvum Opus... Though, I'm sure if they had made a number VIII, it would be that one.

I don't really play any other large series of games, though, so I can't give any more. Maybe with Blizzard games...

Wrath of the Lich King was Blizzard's MO, and Starcraft II was the PO for me.