Hard to tell, most of the game I played don't keep track or are almost a decade old but I'll guess and then add some actual numbers from my Steam account
1. Runescape (Free way to get the MMO grindy experience)
2. Medal of Honor Allied Assault (Bolt action rifles only, 1 hit kills, round based objective)
3. Battlefield 2 (Strike at Karkand Infantry only 64 Player conquest)
As for games on my Steam account with actual numbers behind them
1. Team Fortress 2 (Before they ruined it)
2. Counter-Strike, as a series probably more than 500 hours
3. Fallout: New Vegas
Counter-strike is actually probably my most played series but Steam didn't keep track of hours played when it came out and was therefore the only game I played for about 2.5 years.
And I have over 300 hours on Advanced Wars: Dual Strike on the DS