I think I'd rank the MGS games like this:
MGS1>MGS4>MGS3>MGS2
1, 4 and 3, even though I put them in that order, are pretty simillar in quality for me... with 2 being a little worse than them :P
Anyways, 3 is important because in that game, Zero becomes 1, 1 becomes 2, 2 becomes 10, 10 becomes 100.
:P