I agree with Geraint, its really hard to talk about MGS without giving spoilers.
I have also played MGS 1-4 and more recently 2 & 3 again on my vita, mgs on vita is pretty amazing. I played 1 to death on the original ps1 and mgs 2 has to be my personal favourite, although i do like all 4 i have played. I am still waiting for the ps3 version of the hd remakes to come down in price as the vita version does not contain peace walker and i have never played that one.