Wow, this is a tough one. I include series as a single entry if the games are similar enough, and take into account offspring and spinoffs.
5 - BALDURS GATE 2: SHADOWS OF AMN
I'm a roleplaying wonk, and Baldur's Gate 2 is still the most epic and grandiose of Bioware's titles. Throws almost every significant character, monster and waypoint of Forgotten Realms lore into a huge pot and bakes to perfection. Narrowly beats out Fallout 3 and it's own spiritual successor Dragon Age for the 5th spot.
4 - THE SIMS (SERIES)
Will Wright's virtual dollhouse has blasted through gender and age barriers in gaming to become one of the most successful franchises of all time. Deceptively mundane looking gameplay turns oddly engrossing and time devouring. Excellent gateway drug to get your girlfriend hooked on gaming.
3 - WORLD OF WARCRAFT
Had it been released 9 months later, Everquest would claim this spot as WoW's rightful predecessor, and the true progenitor of the entire current generation of level/loot based MMOs. You cannot deny WoW's impact, however. It has gone past simple success to become a pop cultural touchstone, throttling its own genre with the weight of expectation it has created.
2 - HALF LIFE (SERIES)
Two of the best shooters of all time, the original games serve as templates for how to correctly implement pacing in a FPS. Spawned a host of AAA titles with its flexible and robust engine, including Team Fortress, Counterstrike, Gary's Mod and most notably the deeply funny Portal.
1 - MASS EFFECT (SERIES)
I know people are sick of hearing about it at the moment, but this series has truly become Bioware's magnum opus. A wonderful marriage of RPG and shooter elements that hearkens back to classics such as Deus Ex and System Shock, Bioware's robust characters and cinematic presentation elevate these two games to the exalted status as some of the finest RPGs ever made.
