I disagree; MGS4 was the worst in the series imo. While the graphics, soundtrack and gameplay were good, the story (the most important aspect in the MGS series) was terrible. Also there were a couple of things that just made MGS4 pale in comparison to its previous games. Here are a couple of things that made it the worst in the series:
Drebin Points:
This was a terrible addition and made the game very easy. You can purchase ammo whenever you want, even during a boss battle. It didn't compel you conserve your ammo.
Bosses were just copies and had no personality:
One of the best things about the MGS series is the boss fights, but MGS4 failed to deliver. While the Rex vs. Ray fight and the last fight were good, the B&B corps were just lame. It felt like I faced them before, and there was nothing exciting about those battles. Also they had no personality. You knew nothing about them when you fought them. After you killed them, Drebin will call you and tell you their life story. It's too late for that. Foxhound FTW!!
Lack of codec convos, easter eggs, etc:
One of my favorite things to do in MGS is calling different people with my codec. There were a lot of interesting stuff and conversations between Snake/Raiden with his support team. Like in MGS3, you get a hint that Ocelot is The Boss and The Sorrow's son. Also when you saved, there were some interesting conversations about movies.When you captured a new animal, call paramedic and she'll tell you info about that animal. Sometimes there would be a funny moment when Snake asks how the animal taste.
In MGS2, you got to know about what Vamp's name meant. Also after you killed a couple of birds or slapped a few hostages, you would get a call from the colonel. Those were pretty sweet. MGS4 failed to deliver on that. There were only 2 people on Snake's support team, and they kept repeating the same thing.
Lack of diversity:
In MGS2, you had to take photographs, disarm bombs, snipe, escort, etc. MGS3 - different ways to kill your enemies like using poisonous snakes or shooting a beehive, capturing different animals, grabbing Raikov's suit, placing bombs near the shagohod (sp?), escorting near the end, shooting those plastic frogs, having fun killing those scientists, etc. MGS4 didn't really anything like that. Sure you can choose a side and find different paths, but that's about it. All you really had to was go to your destination, and the game will do it for you.
Terrible story, lack of amazing plot twists, and the answers/explanation were lame:
Nanomachines. That was the answer to everything: why Vamp was the way he is, etc. It was pretty lame, and the game didn't really have any shocking twists. I still lol about Otacon crying over a one night stand. Let's not forget that hollywood type cheeziness when Akiba and Meryl fought the wave of frogs. MGS1, on the other hand, had a lot of awesome plot twists: Master Miller was actually Liquid Snake, the PAL key Snake used activated Metal Gear Rex instead of deactivating it, the genome soldiers, and I have to add the four soldiers who had the stealth camo in the elevator. Epic stuff!
Too many cutscenes:
I know this is a little bit of a lame complain especially coming from someone who enjoys JRPGs and Metal Gear Solid games, but there was way too many cutscenes in this game. There are 10 hours of cutscenes and only four hours of gameplay. I wouldn't complain about the cutscene length if the story was good, but unfortunately, it wasn't imo.
This is how it goes for me: MGS3 > MGS1 > MGS2 > MGS4