Metal Gear Solid: Peace Walker (360):

People call it the game with the most content in the series. This is wrong, it's content is actually on lower side. It's just previously optional non-canon levels (like Snake Tales in MGS2) has become mandatory and endless grind for everything makes it seem longer. Main story is about 30 missions, but extra ops are way over 100.
It wouldn't be so bad if it was optional, but a lot of content and arguably the best content is hidden there. Game hides it's second
and third ending in these extra ops. Monster Hunter missions are all there.
After getting first, fake ending you get credits sequence and that's where the real grind begins. You're encouraged to replay missions for S-rank, play extra ops, collect people for your army, upgrade your equipment, collect design docs and look for a man who manages to escape my base
SEVEN times and runs away to hide in a jungle.

Some extra ops are not that bad and are pretty amusing in their own right (like ghost missions), but it doesn't change the fact it's all the same locations just reconfigured slightly.
Game is designed around the grind and it's impossible to get S-ranks and get everything and everyone on the levels on first playthrough. You simply don't have an equipment for it. Level-1 weapons suck so much you're likely to go out of ammo by the middle of a mission and if you don't have supply marker -- that's the end of a mission.
Only after you get tuxedo, sneaking and battle suits with special bonuses you have a chance. And even then boss battles pretty much require to get max-leveled up rocket launchers to complete them in less than 10 minutes.
Game has a decent variety in it's missions. First is a regular sneaking mission where your goal is to get through the level unnoticed. Your main tools there are tranq gun, close range combat and smoke grenades. Or you do it without gun and just sneak up on soldiers and poke banana in their backs pretending it's a gun and holding them up that way.
AI boss battle missions are pure combat without any sneaking at all. Just take the most powerful weapons you have: rocket launchers and mini-guns. Air strike markers are very effective.
Vehicle boss battles are a mix of sneaking and combat. First you dispatch a squad accompanying a vehicle (preferably without being seen). Even on default difficulty there at 20+ guys so you need to take out squads of four five times at least. Smoke grenades are pretty much mandatory. Then you need to damage the vehicle until a commander notices someone stole his squad and shows up and you tranq him.
All three types of mission have different rewards if you do them the right way. S-rank for sneaking missions, vehicle itself in vehicle battles and mech parts from AI bosses. Vehicles are used in Outer Ops -- yet another skinner box only you not even playing them -- just assign soldiers and vehicles and then wait for next turn to see the outcome.
It took me few retries to realize that AI parts and just "parts" are actually separate and to collect AI boards you need to destroy specific parts, while to collect actual parts for your Metal Gear you need to only destroy AI pod without damaging other parts.
Weird QTEs sequences when you take out AI boards scenes reproduce a scene of David Bowman from "2001" dismantling HAL while he begs him to stop as he loses his identity board by board. It was a cool scene in the movie, but repeating them time after time during first playthrough and then even more later is stupid. AI boards actually become one of the most obnoxious grindy parts because you have to fish for specific boards and they might or might not appear after you spent 10 minutes fighting AI boss.
Gameplay and controls are alright i guess. I just really got soured on them after facing the grind that is post-game. I got to second ending completed half of extra ops and S-rank half of main ops and after 40 something hours i think i am done. I just watched let's play to see post-post-game stuff.
They totally knew what they were doing with designing the game around grind -- there is ALWAYS some carrot hanging in front of you -- for me right now it's a mine that will allow to
Fulton guys much more easily and even reach S-rank soldiers who were previously unaccessible. I am resisting the urge to come back and grind a bit more to see this mine in action.
Story is pretty crappy, just like 4. 4 was heavily relying on fanservice for entire series, while PW is basically does the same just for one game -- MGS3. You're constantly fed with flashbacks and
The Boss worship gets almost intolerable. Ending is supposed to provide a connection between MGS3 and MGS4 but it's clumsily executed. I didn't even realize it was supposed to be a moment where main character finally becomes "villain" from later games in chronology.
Just like with MGS4, setting and extra information is ultimately the most interesting parts of the game. There are hours and hours of recordings with characters talking about geography, flora, fauna and political situation of Central America post Cuban Missile Crisis, theory of AI,
MAD and million other things.
Making all bosses into vehicles and robots was very disappointing. That robs the player of actual interaction with the boss -- what can a tank or walking robot say to you -- he just roars.
Monster Hunter missions were prolonged, too grindy and controls are still not fast enough where combat becomes actually enjoyable. In other words -- very representative of Monster Hunter.
So overall. It's alright. Grind is stupid, but it DOES get addicting and fulfilling to play more to unlock more and more and more stuff. I almost kind wish i played it on a handheld. I also played it by myself and i suspect it's less grindy and more fun to play with a buddy but ah well.
So now i would rank series like that:
3 > 1 (TS) > PW > 4 > 2
Where only MGS3 is a great game.