1. MGS1 - You cant beat the original. It had such a unique feel and atmosphere, great gameplay for its time, and memorable boss fights and characters.
2. MGS3 - Again, great atmosphere, great gameplay, great characters and a story that kept me interested from beginning to end and then kicked me in the balls.
3. MGSPW - Loved the gameplay changes and the portable mission structure. Lots of unlockables and stuff to do. Co-op is a blast.
4. MGS4 - Loved Act 1, 2, 4. Act 3 and 5 were largely forgettable except for their endings. And the overall ending drug on much longer than it should have.
5. MGS2 - I really felt like the series started to drift here. It pulled so much weird shit and presented it in all in such a stupid way. A lot of long, boring cutscenes broken up by short bursts of gameplay.
6. MGSPO - I loved it when I played it but, thinking back, I dont remember much about this game at all.