I really like Quixel a lot. If you are doing any PBR work then something like Quixel or Substance is extremely helpful. Like Armagon said, NDO is fantastic for working with normal maps. If you are interested in a tool that can do both PBR painting and re-topo work, check out 3D Coat.
For PBR and retopology I prefer 3D-Coat. Also a fan of Substance Designer. Quixel last I tried was very unstable (this may change with the new 2.0 release). Topogun I think stopped being developed several years ago (the last update on their homepage was from Oct 2012).