The original Metal Gear Solid is the most successful title in the franchise having sold more than 7M copies.
The sequel sold a mil or two less and Snake Eater sold about half.
The lineup of best selling games on Sony platforms looks like this:
Grand Theft Auto: San Andreas
Gran Turismo 3 A-Spec
Grand Theft Auto: Vice City
Grand Theft Auto 3
Gran Turismo
Final Fantasy VII
Gran Turismo 2
Gran Turismo 4
Final Fantasy X
Final Fantasy VIII
Crash Bandicoot 2
Crash Bandicoot 3: Warped
Tekken 3
Crash Bandicoot
Need for Speed: Underground
Need for Speed: Underground 2
Medal Of Honour Frontline
Driver
Resident Evil 2
Metal Gear Solid
Metal Gear Solid 2: Sons of Liberty
...adding Snake Eater would have tripled the length so i won' t bother.
All three Halo games rank higher than any of the Metal Gear games and 2 & 3 rank above Half Life and Goldeneye...
Metal Gear 4 will sell systems, but it doesn' t have the pulling power or kind of appeal that games like Grand Theft Auto, Gran Turismo or Halo have. It' s just not in the same league...