MGS 3 has fantastic replay value, too - not just the extra camo, weapons and gadgets, but all the plot-related secrets too.

The overall story definitely makes more sense if you've played MGS1 and 2, though. 1 especially. The original MGS still has, in my opinion, the best mix of stealth, action and unique challenges, and the greatest characters. Unlike MGS 2 and 3, every major villain in the game has a big role in the plot rather than just being snake fodder, so to speak.