depends what you want to do, or how advanced you are? I am guessing your just getting into it. I would have to suggest Z-Brush. it's not cheap, but it does everything as far as modelling is concerned. I don't think there is any movie or game studio that doesn't use it. From the making of LORT movies, to you name it. Now that is for straight up modelling. If you want something total that does modelling / rigging / animation etc. then I thing the industry standard is MAYA by autodesk, who also makes 3DS MAX, which is also another industry standard. Most if not all game and movie studios use those 3 if not 2 programs together. Z-Brush and Maya.
If your really really new, and want to just "play" with 3D, then wings3D is a freeware program that allows box modelling and polygon modelling. Then there is Poser, which is bundled with free models, and there is a huge market place to buy models and get free models via some sites like DAZ3d, and Renderosity.com. I don't use poser much, and not in some time, but even PoserPro has modeling within it.