In my possession at this very moment is every single non-portable Metal Gear (including 8-bit and special editions ie: Substance/VR Missions) AND every single non-portable Splinter Cell.
My vote is for Metal Gear.
I was a little let down by MGS4, though. When I get my hands on Conviction, I can give a more accurate comparison. Historically speaking, I gotta stick with the Snakes (Solid, Naked, Old, Raiden)!