Kinda depends on what you're doing and how you like to work.

Generally the easiest way to do organic modeling these days is to get one of the sculptor apps like ZBrush or 3DCoat and build your model in there, but this requires that you have a decent Wacom tablet and are good at using it. Good understanding of form and volume also helps. If this is not your cup of tea, then there are also apps like Modo or Silo or Hexagon etc which are dedicated modellers.

I would also suggest trying out the demo/trial versions of as many apps as you can to see whether any of them appeals to you.