MGS1 was rather straightforward. It had some plot twists, but overall it wasn't that hard to follow the narrative. Yeah, some characters weren't who they said they were, and maybe you misunderstood the villain's plan, but it all makes sense.
MGS2 was a postmodern marvel, full of inexplicable phenomena, with the final act being a bizarre, surreal experience that turns the entire previous act on its head. But there's a method to that madness, and the game doesn't behave in an erratic way for the sake of it; in a very meta way, the game uses itself and its own narrative to tell its story. It's really wonderful.
MGS3 is fairly normal, plotwise, for 95% of the game. It goes off the rails in the final moments, changing the meaning of the whole game and presenting conspiracies everywhere. Out of the first 4 numbered titles, it's my least liked one, although it is still good (except for the Virtuous mission and the first part of Operation Snake Eater, I hated the jungle),
MGS4 is... well, MGS4. Can't think of any other way to describe it. Take the madness of 2, add a few conspiracies from 3, bring back some characters from 1, turn it all up to 11 and then drop some acid while you listen to Foo Fighters doing a drunk cover of Godspeed You! Black Emperor. Well, the final product is a bit crazier than that, but you have a pretty good idea overall. Overexplaining everything, coming up with the most ridiculous excuses for the already ridiculous events in 2, showing cutscenes for hours at a time, it's a hell of a ride. And if you let yourself go, this becomes an incredible game that will have you questioning your own sanity. Not as in the game manipulating you (which was the case in Sons of Liberty), but in a "did this really happen?" way. It's a smart, stupid, awesome, delirious, cool, embarrassing game, and I love it so much it has become my second favorite game in the series, after MGS2. I love it unironically, mind you. It's that bad in the right way. It's that good.
Haven't played Peace Walker or V yet, so I can't speak for them. Portable Ops was terrible, and I dreaded every second I spent with it.
