It's damn near impossible to decide on ONE ABSOLUTE video game being the best of "All Time". At any given moment it could change depending on what mood you're in and what the game is doing for you.
At present for me it's a toss up between
MGS 3: SE and
KILLZONE. Both games do just about everything for me, with the exception of annoying me when I want to do particular things that isn't allowed in the games. Neither game will let you strip the dead of their gear and weapons, which makes no sense what-so-ever. *YEAH, I'm sure Snake wouldn't have sliced a dude's throat open, then stripped the dude of his uniform complete with hood and mask, + the dudes AK-47 upon encountering the very first grunt in the woods to where Sokolov was being kept.
Traditonally though I'd have to say...
Shock Troopers, I just never get tired of playing it.
MERCENARY X99 