Metal Gear Solid: Substance The substance of this game is enough to keep you playing
Even though you don't get to play as Snake (well for the first 20-30mins) the story unfolds and you learn to love Raiden.
This games story line keeps getting better and better. The sinking of a tanker makes it neccesary to build a cleanup facility known as the Big Shell. But later in the game you learn there is more to it than that. It even builds you up eagerly awaiting for MGS4 (as MGS3 is a prequel).
The dog tags to collect to unlock everything on this game supplies it with an extremely long replayability factor.
This game also has 350+ VR training missions to test every ability Snake and Raiden have.
Snake Tales are also on this game where you play as Snake on Plant.
Compared to Sons of Liberty this game has a few added extras but not really worth paying loads for it.