ok, god spells vary in power depending on the target of the spell. Claws of Guthix lowers defence of the target by 5%, and so is good for high defence monsters/players, such as mithril dragons, or the barrows brothers Flames of Zamorak lowers magic level by 5%, and so seems pointless against many monsters, and only useful if you're getting barraged and want it to stop :P Finally, Saradomin Strike lowers the opponents prayer, and once again seems pointless against monsters, since many of the praying monsters have infinite prayer anyway, but can give the edge in a PvP fight. So, from this idea, i find claws of guthix the best spell, especially since there is the option of the void knight mace autocast, yet the other two must be manually cast. On a last note, i also find charge ridiculously useful, as without it, the god spells are too expensive for their max hits of 20.
Good Luck, whichever you choose,