You should play all the main ones. Also Peacewalker if you can. Since you started with Ground Zeroes maybe you should play MGS 3 then Peacewalker then 1 > 2 > 4.
Chronology goes like this: MGS3 > Peace Walker > Ground Zeroes > Phantom Pain > The original 2 MG games (You can just read about those) > MGS1 > MGS2 > MGS4.