Get the one you like. Most will say you should start with something simple, which means a caplock as opposed to flint. But if there's something that really turns your crank, go for it.
Voodoo is right-- depends what you want to do with it. My first BP gun was purchased for a single-minded purpose-- to hunt muzzleloader season. That really, really, narrows the choices down. You start with your local hunting restrictions and go from there. In my case it meant a rifle with a sidelock of at least 50 caliber, etc.. The one that was on the wall at the local gun store was the other limiting factor-- he had one that met the hunting criteria. My choice was thereby reduced to one, so it was easy.