The first one is the best place to start. It also has the best story out of the four main games. If you start with any other games in the series you'll probably be totally lost as far as the story goes (and if you don't care about the story, you're playing the wrong series).
I would say play them in this order 1,3,2,4 since the first explains who the heck snake is and the third is a prequel. The second has you playing as a different person, and the forth... i just say save the best for last.