Since GalCiv 2 and later games have included cheat codes, probably the one I use the most is CTRL-N to create a new start if I don't like the one that I have. However, there is one command that I've always wished for, and never had... an ability to increase the AI difficulty while inside a game.
Many times while playing GalCiv2, I would start the computer at the level just above challenging (where the AI gets full control of itself and 5% econ bonus), CTRL-N to a good start, expand in the colony rush, and then turtle myself, while creating a great empire. Eventually, I would be at the point where I'm ready to strike out to the galaxy and destroy the rest of the galaxy.... However, sometimes, I had too GREAT a start, and could totally destroy everyone else at this point without a problem... What I always wished for was the ability to have just a smart AI during the start of the game, but be able to get a cheating AI once my empire was set up.
I hate losing the game when first exploring, but I would LOVE to lose a game once I'm set up. Since the primary purpose of cheat codes is to make the game more fun in single player, it seems to me like the ability to increase the difficulty while inside a game is a Win-Win, and I don't personally see any down side, since it is optional.