Most definitely WarCraft III: Reign of Chaos

Played the story countless times, and Custom Maps on Battle.Net... I can only imagine the hours wasted away.

DOTA, DBZ Tribute, Poker TD, the dozens of Mauls (PokeMaul was my favorite!)... so many awesome games!

Going with C&C Red Alert 3 based on my Steam list it gives me 228 hours of gameplay, TF2 I imagine would come in second but I hardly play that game anymore and 3rd would be Planetside 2 and 4th left 4 Dead 2.

I've probably played a lot of games in the past for much longer but those 4 are probably the most recent since 2008.

Runescape, League of Legends and Call of Duty

Played more then 1000hrs on each of them for sure

Counter-Strike....just way too many hours to remember.

On Consoles: Probably Fire Emblem: Radiant Dawn. Hyrule Warriors is approaching with high speed though.

On Handhelds: Either Tetris or Fire Emblem: The Sacred Stones

On PC: Master of Orion 2: Battle at Antares or Wizardry 8. Both are over 10 years old and still play them very much more than anything more recent. Probably way over 3000h each

Super Mario 3D Land was an unexpected time sucker. I clocked over 300 hours. Of course, Smash Melee was up in the 1000's.

Think the same. I think anywhere between 1000-4000 hours. The thing is I am moderator on a server, so I do idle a lot. However At least 1000 hours would be safe to say how much I played. 

Call of Duty 4.

Had over 25 days worth of game time, pushing 30.

Edit - A lot more, as this is just online multiplayer leaderboards.