MGS4 is when I stopped playing that craptastic series. I didn't even finish it, I think I stopped like half way. I should have stopped after part 2 sucked. I never played part 3 because part 2 blew, but I heard part 3 was actually very good. I only gave part 4 a chance because it was the first one on the new console at the time and graphics were unreal.
Part 2 was bad but not unplayable bad.
MGS3 was great story (at least as far as MGS goes) but ugly low poly jungle graphics.
MGS4 was tarnished mainly coz of too long cutscenes. Otherwise, rest was decent.
MGS Ground Zeroes was GOOD. Felt like a different, somewhat harder MGS.
MGS Phantom Pain, never tried, despite paying full price at launch. Would have sucked too much time with base building and open world gameplay and besides, it's unfinished.
MGS Revengeance, couldn't get past the very first obstacle (a stupidly hard Metal Gear). Not made by Kojima.
MGS Survive, not made by Kojima so what's the point? A turd, judging from reviews.
Metal Gear Ghost Babel. Great gameplay, for a handheld game.