Definately gray because of its versatility. For examply, say you go for an interview at a very conservative firm, such as Goldman Sachs. The most conservative suit is a gray one. Gray is definately the way to go as your first one. Then you can get whatever, such as black or navy or pinstripes. Just make sure the suit has 3 layers: the outside wool fabric, the inner silk lining, and the "skeleton", which gives it its structure, which is in between the lining and outer fabric. As long as you follow theese guidelines, you're good to go. Also, with your first suit especially, stick with a 2-button. It's more classic than the 3-button, which is a bit dated right now.