At least among the PS2 MGS entries, I think it's the easiest game among them to just pick up and play. I like the survivalist theme, and, unlike the other two PS2 games, a lot of the game takes place outdoors. I found that it kept the stealth gameplay fairly fresh.
The story, such as it is, is stand alone in MGS3; plus, it doesn't have quite the amount of ridiculous cut scene time found in MGS 2 and 4.