This comes up regularly and I wonder why. MGS2 is a MGS1 clone with a lack of interesting characters and a really fucked up over the top story. Kojima stories are always kind of cringy, but this one takes the cake.
Personally speaking, it's the worst of em all. Well, apart from MG5 obviously
I vote for Solid because of MGS1 beeing a revolutionary masterpiece and MGS 4 beeing a underrated grand finale.
MG3 is a masterpiece hands down. But Peacewalker and Mgs5 drags it down for me.