Well, Mgs1 was HARD! Mgs2 was soo easy, Mgs3 was easy but still hard lol, and Mgs4 if you're playing on a decent difficulty it will challenge you.
BTW Fallout 1 was very easy in comparison to fallout 2, fallout 3 was too easy, seems like they don't know how to make a survival game, enemies shouldn't have that ridiculus amount of health (later in the game some enemies are like shooting at a brick wall for several hours), it shouldn't be like that, in a true survival game enemies should die from tops 2 shotgun shells in the chest, but ammo shouldn't be found everywhere, right now my fallout 3 character has over 20.000 caps and 1000 shotgun shells and I never use melee, they fucked up in making a survival game, it's a fps shooter!