Counter Strike.
I've been playing since 2001, made the jump to CS:S a year and a half ago and I still play weekly. It has everything I need in a shooter: a widely varied community, good fundamentals for a shooter (though the original had better fundamentals), a wide range of great mods, easy access to servers through Steam, and it's just friggin fun.
Team Fortress Classic is up there. Quake 3: Rocket Arena left me with many fond memories. Unreal Tournament (especially low grav instagib) kept me occupied until I couldn't find any more populated servers. Other contenders are Everquest, Guild Wars, and Planetside.
In the offline play category I'd say Warhammer 40k: Dawn of War since I've played and enjoyed it since it first came out.
Contenders are: Company of Heroes, C&C: Generals, FF7, Half-Life 2, Half-Life, Warcraft 3, and...I don't know, Giant Citizen Kabuto.