Got eight series but over 23-28 games in total depending if you count the WoW expansions as different games. Ranked from most played to least play even though it's pretty even.
1. World of Warcraft 1-6
2. Elder Scrolls 1-5
3. Diablo 1-3
4. Eye of the Beholder 1-3
5. Knights of the Old Republic 1-2
6. Baldur's Gate 1-2
7. Fallout 1-4
8. Phantasy Star 1-3