5: Number 5 on my list is the Fallout series. Fallout originated on the computer and only has 2 games on Xbox, but there is no denying that the series has a huge following on Xbox. Fallout 3 is made by Bethesda and Fallout: New Vegas is made by obsidian, and those 2 companies are basically 2 divisions of the same company.
4: Number 4 on my list is the Elder Scrolls series. Elder Scrolls, made by Bethesda and obsidian. Just like Fallout, most of this series' games are on the computer, and there is 2 games from the series on Xbox. The games on Xbox are The Elder Scrolls III: Morrowind and The Elder Scrolls IV: Oblivion. this series has a lot of followers, but not as much as the next ones on the list.
3: Number 3 on my list is the Mass Effect series. Mass Effect is made by BioWare. Mass Effect also has 2 games for Xbox, Mass Effect and Mass Effect 2. The mass effect series has a huge, almost cult-like following, but it's following pales in comparison to the next series in my list.
2: Number 2 in my list is the Halo series. Halo is made by Bungie and they have 5 games in the series, all of which are available for Xbox. The games that have been released in the series are Halo: 1, 2, 3, ODST and Reach. Reach is the last game in the truly historic series that was, and still is, loved and played by millions.
1: Number 1 in my list is the Call of Duty series. Call of Duty is made by 2 divisions of Activision, Infinity ward and Treyarch, Infinity Ward making the even number games, and Treyarch making the odd numbered ones. Activision has released 7 games in the series, Call of duty: 1, 2, 3, 4 Modern Warfare, World at War, Modern Warfare 2, and Black Ops. I cant think of a single person who has played a game in this series and not enjoyed it. Despite a recent lawsuit involving the 2 divisions of Activision, there is still hit games coming out of the series.