MGS the PS1 video game would make a good movie, with some adjustments. MGS the series is a giant mess of a story and I don't think it would ever work.
Apparently they wrote a script - who they are, I don't know... But I do remember reading a mag in the early 2000s that said that Ben Affleck was being lined up for the role. In fact, given how well he did with other endeavours, it could either be a flop or a hit... If he does well with Batman I do think he would make a good Solid Snake.
Anyhow, it was probably some shit lads mag that I read whilst having a shit, but again... How could you cram enough juice into a 2-3 hour film to appease both fans and cinemagoers?
It would all come down to character-cram, as in, a kind of Marvel-lous mashup of characters, much like Winter Soldier - a blend of actors to share as much screen-time as possible in order to drive the plot forward and resonate some kind of meaningful story. In reality, again, it'd have to be a serial deal - which again, carries with it more risks and pitfalls.
Long story short - it has to be the hardest source material to adapt and Metal Gear Solid would be the crown jewels of adaptations.