Well no series is really a "must buy", everything is subject to review and decision making. And I don't buy anything at launch or pre-order unless it's CD Projekt, but that's almost more out of principle.
Anyway, here's my list in no particular order:
-XCOM
-Gears of War
-Half-life (lulz)
-GTA / Red Dead
-DiRT & DiRT Rally
-Fallout
-Elder Scrolls
Things that were on the list but have since dropped off, in case this interests anyone at all
-Gran Turismo (ruined by developmental corner cutting, such as re-using lower poly models, and crazy micro transactions)
-Resident Evil (I'm from the other camp that actually enjoys RE6, and not the shift to first person in RE7, although it's a pretty good game but still. I will miss Mercenary mode)
-Diablo (always online, piss off)