I also enjoy the Fireball as a signature spell. I've looked at it and I do think that Hurl Boulder makes a better finisher....but some people just like to watch the world burn. Besides, Fireball doesn't do much less damage up front, and will usually do more damage in the long run (if the finisher fails, but that's not really what you want to happen with a finisher).
I also like Agony a lot. I've even put it in my Priest book at one point. It's not in it right now, but it may be again in the future. It's just so nice against Forcemaster and other multi-attack opponents.