I have indeed read the question wrong, however:
How can you spend hours and hours on half life

. Or most SP games for that matter.
I spend thousands of hours on transport tycoon as a kid, hell, when I got it I was 7 iirc or so I got up at 5am just to play the game

for a while after I got it. And I ran home after school exited to play TTD again

.
More recently, I've spent quite a lot of time in civ4 games and before that civ3 games.
Used to play age of empires 2 for 4 or 5 years in a clan, and age of empires 1 a lot on SP before I got aoe2, I loved the scenario editor & making my own battles. And I SOOOOOOOO hated the bloody population limit @ 50

.
I played mohaa for ages every night when I discovered MP on it, was hooked for ages and sure I have more than 1000 hours on it.
C&C series: Not really, until Generals that is, I simply finished RA and RA2 twice and had a few skirmishes but most time went into skirmishes and online in Generals.
Most modern games aren't that addictive as games used to be unfortunately

.
If the poll is about time played, why aren't the gta series in the list (they should be in too if it was a poll about the best games) ? Sooooooo much hours of VCMP, MTA: SA and SA: MP there for me. And sp too off course, finished vice city many many times and san andeas too.