6. Counter-Strike: Source
Weapon-purchasing system and variety, customizeable maps mixed in with excellent staples, ease of access and a robust online system full of servers for almost every map and game type. That is why Counter-Strike: Source had the staying power in the online PC shooting game world. The tournament game of choice, being good at Source meant you were good at video games. Everyone and their mothers (who were opposed to violence) played vanilla Counter-Strike; source made it legitimate.