DMC1 is very... old. It's when cuhrazy action games games were still discovering themselves. It's good, but it's very simple compared to the others. 2 is bad, don't play it. 3 is considered the best by a lot of people, and 4 the best by the rest. 4's got a more complex fighting system than 3 with style switching and all that jazz, but 3 does the story and atmosphere better. And the bosses are a lot more fun in 3 imo. Them Vergil fights, baby.
So to answer your question, probably 3. It's technically set before all the other games anyway, so you're not missing anything plotwise.