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I just got into the metal gear solid series this summer. I beat these at least once already to get the rank. I found the 3rd game the hardest to get the best rank, although one of the easiest games on normal mode without going for ranks. Amazing series.
PSP:
Metal Gear Solid - 1 (PSN download) - Big Boss rank, Extreme mode
Metal Gear Solid - Portable Ops
PS3: Metal Gear Solid 4 - Big Boss Emblem -The Boss Extreme Difficulty
PS2:
Metal Gear Solid 2 - Big Boss Rank - Extreme Mode
Metal Gear Solid 3 - Markhor Rank
Metal Gear Solid 3 - Foxhound Rank - Extreme Mode
PC:
Call of Duty: Modern Warfare - Veteran Difficulty
Call of Duty: Modern Warfare 2 - Veteran Difficulty