I chose this list to limit myself:
en.wikipedia.org
Campaign completed main entries only, so the likes of Wolfenstein, Megami Tensei, Phantasy Star or even Castlevania are out of the running due to bad availability. Some went to shit as Leisure Suit Larry, Ninja Gaiden (remember Yaiba?) or Contra but I am willing to overlook phone and online-only games. Especially if those never left Japan:
Doom
Dragon Quest
Final Fantasy
Fire Emblem
Mana
Mario
Mario Kart
Metal Gear
Metroid
Tex Murphy
Mortal Kombat
Street Fighter
The Legend of Zelda
Ys
I have no life and I must scream.
The Oregon Trail is the longers running game series btw.
PS: The list only has series that are older than
25 years. So no Ace Attorney, GTA, Ratchet, or Yakuza which I all finished. An actual listing of all the series I finished would shame even me...