One has to be careful with selecting propellants, solid or liquid, purely on chemistry. There are propellants that on paper seem to be really great. Liquid fluorine, for example, has a LOT of energy on paper. But it has a few disadvantages over liquid oxygen, like being hideously toxic, being hypergolic with almost anything you can think of, and when burned with hydrogen, producing the delightful HF as an exhaust product.

With solid propellants, you are mixing your fuel and oxidizer together. You want materials with a lot of energy, but that are relaxed enough to be comfortable in close quarters, too. You don't want a solid rocket motor that will blow you an Olympic-sized swimming pool if you accidentally whack it with a wrench.

Nitronium perchlorate is mentioned on this useful reference page. I'm not that well-versed in solids, but it sounds a little too sporty to me.