From Best to worst:
1. Metal Gear Solid 3
2. Metal Gear Solid 1
3. Metal Gear Solid 2
4. Peace Walker
5. Metal Gear 2: Solid Snake
6. Ground Zeroes
7. Portable Ops
8. Metal Gear Solid 4
9. Metal Gear
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I'll go into details later on, but I do think MGS2 has the best gameplay in the series (so far) and great mission variety. MGS1 & 3 have a more gripping story imo.