IF you can get them, then Dynamit-Nobe/RWS #4, as mentioned already, are the hottest I've ever found - they will, of course need to be top-hat or musket caps, rather than the smaller versions used for handguns and some rifles. There are other brands, but when it comes down to it you'll be stuck with what is available.
I didn't know that CCI made musket caps.
BTW have you noticed the other recent arrival from Norn Iron? Lives somewhere near Portrush, it seems. The Creedmoor team is building up nicely!!