When I first went to metal miniatures, I started with 15mm ACW figures. I started studying the Civil War in detail in the early 90s, and I've amassed a huge library, both of scholarly works and primary sources. My first ever miniatures game, way back in 1997, was Fire and Fury. It's my favorite period to research, paint, and game, and whenever I hit a painting slump, some ACW figures are sure to pull me out of it.
My 15mm armies have figures from a wide range of manufacturers. I started with Musket Miniatures, added some Old Glory, moved up to some Battle Honors, then filled out a few units with Peter Pig, Stone Mountain, Frontier, Battlestandard, and Minifigs. Each range has its own appeal, but the brand I just keep painting is Old Glory. They seem to me to have the best period flavor of all the lines.
These Union cavalrymen are all Old Glory sculpts, a mix from packs ACW31 "Union cavalry with pistols and carbines" and ACW16 "Union cavalry with sabers."
The cavalry regulations called for horses of the same color for each squadron. I doubt this was ever acheived, but I like the look of it. I used two tones on most colors, giving the figures a little extra depth.