plain and simple, Counter Strike: Source

i stayed up 16 hours one time playing it...had about 20 cups of coffee (lmao)...talk about bladder problem

but yeah, runners up would be
Age of Empires II/Mythology
Half-Life 2
CS: 1.6
Serious Sam The First Encounter

and probably lots others haha, but #1 is CS:S

luckily my ownage rig can play it