My experience with a sidelock & #11 caps is that real BP is easiest to light. Of the subs I'd choose Pyrodex as the easiest to light consistantly. With most of the rest I've experienced hang-fires with about all of them with even the hottest #11 caps. An occassional hang-fire with Pyrodex, but better than most other subs.
When I shoot anything but BP in a sidelock I use a Hubbard Mag-Spark 209 conversion nipple. It replaces the #11 nipple & uses 209 primers. Musket cap nipples are also readily available and might be better than #11 caps?