My favorite miniatures for Civil War figures has always been Old Glory. There are the obvious things to like about them, such as the quality and the various poses. But what I like most about them are the pieces that seem like they're merely decorative, but they are so interesting that I had to make them into relevant pieces for the game.
For instance, the signal tower is a piece they constructed. This has no game value for any other rules set, but I liked it so much that I added it to the 1862 rules. Players are able to send orders over great distances using this piece. I've also used the civilians in the Bull Run scenario, their photographer for Matthew Brady, their dead pieces for bodies, and several others. Most impressive are their generals. They have packs of figures molded to look like specific generals. These are perfect for the generals you choose out of the Command Combat books for your armies. If you have generals they haven't specifically sculpted, many of their non-named generals look close enough to whomever you want to field.
You can find their Civil War figures at: