The original MGS is my favourite one, though I think it comes behind MGS3 in terms of technically how good the game is. It's hard to overstate just how much of an effect the original had on future action games. I personally think cutscenes
like this still hold up today. I just wish it was a bit longer and had less Resident Evil-style plodding around at the end of the game.
If MGS3 was the zenith, the fourth was where I lost interest. I seem to remember being one of the few on these boards who wasn't a big fan of it at the time, though it appears the general sentiment has gone the same way over the years. It was a very fun game to play, but that was about it. The plot started off farfetched and became increasingly ridiculous as the game continued, with central plot points around which previous games based themselves being contradicted or handwaved with 'muh nanomachines' in single sentences before moving onto something else. The 'stealth' bit was thrown out of the window, more included as a 'yeah I suppose you could sneak through this area, but DON'T YOU WANT TO CAP SOME FOOLS? HERE IS A LIGHT MACHINE GUN' kind of thing. The character of Snake also completed its flanderisation from
James-Bond-meets-John-McClane badass to
complete wimp.
I haven't played MGS5, and actually haven't heard much about it.