Deliverator
Graphical Hackificator
For 3D Leader Art you should use Blender And Deliverators Civ Nexus 6
@sukritact also uses the paid-for tools ZBrush and Substance painter I believe. He talks about his process about in this episode of Modcast:
http://civcomm.civfanatics.com/polycast/modcast/season9/episode73.mp3
@ChimpanG describes his process for making 2D Leaders by making a 3D model:
"I use quite a few. Daz Studio to pull a base model to work with, zBrush or Hex (mostly hex recently because I'm crap with the zBrush interface, and I like to edit vertices directly) to manipulate the mesh, create new meshes for attires and have them conform to the base, patterns and stuff in vector format in Adobe Illustrator and then into Photoshop to create texture maps (diffuse, metallicty, bumps, normals etc etc), then pull it all together, pose, set up a lighting rig and then render in 4k"
"just find a method that works for you. They all have their ups and downs. Some things are easier one way and harder another, and vice versa. I only started doing it this way because I'm atrocious in Photoshop, certainly nowhere near the level that others here can produce. If you're more comfortable with a brush in your hand, then photoshopping will probably be the better method for you."
For 2D leader Art you should use Gimp, Paint net or photoshop
You will need the .dds plugins for Gimp 2 and Photoshop to open save textures. Paint.NET can open some .dds textures but not all.