Like Dan with the original MGS, this was the first one I played, and it had the same effect. It made me realize the potential of games as an artform with its impressivie cinematics that made me feel as if I were watching an awesome movie. With that it cemented me as a "real gamer"--before that I played only baby Nintendo games (sarcasm).
Objectively the best one with a relatively more grounded story, added gameplay aspects like survival, not to mention some the most memorable boss fights in gaming.
3. Twin Snakes
To me, this is up there in quality as the Resident Evil remake. I tried going back to the original MGS after playing this first, but couldn't make it past the first Ocelot battle.
I almost didn't finish this one, but in the end it was worth it for all the ridiculous moments.
I've played Ground Zeroes but I'd rather not compare it to these fuller games. I'll wait for the Phantom Pain before I do that.
Also, I can't wait for the upcoming MGS Endurance Run. :P