RChu1982: For the Alchemist, Psionic, and Mage, it's no contest: Staff of Doom all the way.
In the Alchemist case, however, you need to weigh the Staff of Doom against the alternative, which is to use a shield and not care about melee damage. You can use a sling for when you want to attack, but switch to a shield when you're casting a spell. Also, there's that Flame Staff, which isn't that much worse damage-wise, and isn't cursed so you can still have a sling or shield in your other slot. (Just mind the 20% Blind.)
Psionic and Mage can't use shields, but can still use the Flame Staff.
(There's only one Flame Staff in the game, but it's a guaranteed drop from a unique enemy.)
RChu1982: For the Priest and Bishop, they get the Mace and Flail weapon set. While whips (Bullwhip, Cat O' Nine Tails, and Vampire Chain), can hit from extended range, they only get half the Strength damage bonus (while allowing you to equip shields, to balance).
Worth noting that they also only get have the Strength *penalty*; this is very useful if you're using a full caster Bishop (Int/Pie) or a fast caster Bishop (Int/Spd).
RChu1982: Some of us, including me, want to create Battle-Bishops. Max Strength, Intelligence, Speed, and Senses (this is done by level 31, as a Human). By doing this, I get PowerStrike, PowerCast, SnakeSpeed, and Eagle Eye, creating Bishops that are (at least decent), at melee, ranged, and magic combat.
The way I see it, "level 31" means never (assuming a full party).
In practice, a battle Bishop will get Power Strike and Power Cast in the teens, but likely no other expert skills before the game is over. (Levels are 12/17 for a human, something like 16/18 for a dwarf, and levels may vary for other races; I note that lizardman is worse than dwarf for this build.)