Power (more hitpoints, strength etc. for caster), Magic power (+1 to all magics, max 10), Death, Armageddon (be careful with these), provinces, dominion, ANY monster or unit in the game (including sphinx, nataraja, blood slave etc.), blood slaves (notice -s!, check the one before...
), magic gems, food, army (only militia)...
You can wish for these, but you maybe shouldn't...
Experimenting can yield surprises.
If you are playing to win, wish for Doom Horror or Vastness and cast Gift of Reason on them. They move 10 provinces at once and fly, so they are the ultimate SC for making your enemy mad and devastating his backlands. They both have Blood Vengeance and high stats.
If you have to defend a particular province you can wish for a sphinx and block the castle gate...