Metal Gear Solid:Peace Walker has already been rated. It's rated T, for teenagers, ages 13 and up.
The ESRB has rated Metal Gear Solid 2 'M' for 'Mature'.
The best Metal Gear Solid game is what you define as 'best'. Personally, I think Metal Gear Solid 4: Guns of the Patriots is the best Metal Gear Solid game, it is also the highest rated Metal Gear Solid game ever to be developed and published.
It's rated 'Mature' (17+).
The Document of Metal Gear Solid 2 - 2002 V is rated/received certificates of: USA:M
Metal Gear Solid VR Missions - 1999 VG is rated/received certificates of: Australia:MA USA:M
The 1998 PlayStation game Metal Gear Solid from Konami is rated M for mature by the Entertainment Software Ratings Board (ESRB) for the following reasons: animated blood and gore, animated violence, and mature sexual themes.
Metal Gear Solid Peace Walker - 2010 VG is rated/received certificates of: Australia:MA UK:15 USA:T
Metal Gear Solid The Twin Snakes - 2004 VG is rated/received certificates of: Japan:15 UK:15 USA:M
I think it's for blood and violence and strong language
Metal Gear Solid - 1998 VG is rated/received certificates of: Australia:MA Australia:M (original rating) Germany:12 New Zealand:M UK:15 USA:M USA:M
'Metal Gear Solid 2 Sons of Liberty' - Making of the Hollywood Game - 2002 V is rated/received certificates of: UK:15
It's rated "M"