This is the worst game in the series due to poor story and plot development as well as an unbalanced game design, proving the game was released before it was finished.
Although the core gameplay mechanics are great, the main story missions place you in many situations where the rules of the game change unexpectedly. There were a handful of missions starting in the African phase of the game where Snake was required to chase down a vehicles, or try to find a prisoner, or assassinate a target with little warning as to what was needed to attain success in the mission. Up to this point, stealth and intelligence gathering were key to success. But in these missions, speed and accuracy (and in some cases the need to go back and develop weapons) were key. The rules changed. In some missions, enemies can automatically spot you, and in others, the difficulty was turned way up (not that this is a bad thing, but it was done randomly, with no warning).
Even so, the main missions are mostly good solid tactical espionage action, but a few bad eggs clearly show the game was released before it was finished.
The story however is so abysmal the whole game suffers because of it. The pacing is off, the characters are irrational, and there is no climax. The customer was sold a game where Snake becomes the villain, yet this game offers nothing villainous! Snake kills Skull Face who deserves to die. He kills some Mother Base soldiers because if he doesn't a world plague would break out devastating the whole earth. He rescues child soldiers from war. None of this is villainous. The story made no sense.
Then there is "twist" at the end. You come to find out that you have been playing the medic from Ground Zeroes the whole time. You are not even Boss. You are just pretending to be Boss. It is MGS2 all over again. The only value this adds to the franchise is that it explains why Solid Snake kills Big Boss twice in MG1 and MG2. Pretty lackluster motive to play 90 hours and a poorly executed plot.
I do not recommend this game to anyone. You can find out everything there is to know about this game from the trailers (which show everything in the game, btw. If you have seen the trailers, you have seen every cut scene in this game). It is not worth your money or your time.
Finally, the game is missing its last mission. Look up Mission 51 and you will know what I am talking about. The game is clearly unfinished.
Kojima himself is the man who sold the world. He sold the world a game in which we see Snake descend into villainy. Yet this game does not offer a character who is villainous - or even Snake for that matter! Kojima sold us a bill of goods. A gimmicky lackluster poorly written poorly executed game deserves the same score. I give it a 5 out of 10.