If I remember right, you could get to level 7 in dom 1, but 7 didn't do much for you. You could do level 5 prophet with the sword of justice (I think that's the name, it was the two handed artifact that increased priestly power by 1).
For prophets, I always pick a strong combatant at the beginning of the game. Unless you have a combat pretender, your prophet is your best general in the beginning game. In the late game it can be lots of fun picking a prophet. Gift of Reason plus high end summoning can give you Iron dragon prophets or abomination prophets. I've been speculating that some of the Ryleth void criters could be a fun prophet
If I'm in a tough game, I like using a wraith lord rainbow mage as my prophet. Pretty tough, makes a good rainbow mage, and if you keep him in your dominion you'll never loose him.