The best editor/construction set/utility you've used?

I would have to say... The Elder Scrolls Construction Set, Photoshop and HMT.

I have both Gimp and Photoshop, but I rarely use them.

I have nearly every image manipulation package available. Gimp can be quite scary especially if you are previously used to using Photoshop.

Here are some tutorial websites to help you. They also illustrate some of the features of the Gimp program.

http://gimp-tutorials.net/
http://www.gimp-tutorials.com/

Modeling - Blender
Diffuse Textures - Paint Shop Pro 9
Normal Maps - CrazyBump
Mod Creation - TES4 Construction Kit or Fallout 3 GECK

Modeling - Autodesk 3D Max, for some reason it doesn't like nifskope (Extra key given to me by uncle's architecture firm)
Image - Photoshop CS4 (Obtained from same source :D)
Mod tools - Battlefield World Editor. Everything could be modified directly from the program, including textures and simple models, although there was slight control, and entire new maps and structures could be created. The program was bigger than the game!!! Roflmfao