Most realistic, in 3D, is Blender. DOWNSIDE: Blender takes a LOOOOOOT of skill to master, you practically have to make it your life, but if you want to produce images as good as Disney Pixar, that's the price!
Gimp is a raster editor and INkscape is a vector editor. There are ways to get realistic results in either one, again with practice, but not impossible. That being said, Gimp and Inkscape together are easier to learn than Blender, and Inkscape is also easier to pick up than Gimp.
Why not get all three? There are awesome things you can do by using all three powers combined that you couldn't do as easily individually. For instance, draw up a geometric shape in Inkscape, export to PNG, open the PNG in Gimp. run some filters on it, resave it, open Blender and map the PNG onto the surface of a transparent sphere, so you have a cool curved logo... that's just an example.
Check the site I've provided, this guy writes tutorials on all three!