In CRPGs where you control an entire party of characters, people usually balance out the party with at least one fighter, one mage, and one cleric. In Dungeon Hack you don't have that luxury but you do have the next best thing: multiclassing.
Here's why the Fighter/Cleric/Mage is the best choice for the game.
You'll be able to use any armor and weapons when not actually casting spells.
Healing Spells: Not quite as usefull as one might think, but still pretty good.
Create Food: VERY useful. Makes the game much easier since food is so difficult to come by. While starvation doesn't actually damage it makes it impossible to rest meaning you can't rememorize spells and are very dependent healing shrines. Once you can cast it, which is relatively early in the game, the Create Food spell takes starvation completely out of the equation.
Spiritual Hammer: VERY useful. Since the hammer returns to you after it hits a monster or a wall, you can do some rapid fire ranged damage if you've got one in each hand. This is extremely effective against monsters in the beginning of the dungeon, and slightly less so in the middle. For the more difficult beasties the spiritual hammer can be used to soften them up before coming in for the killing blow with a powerful melee weapon. Also, since the hammer is a magical weapon it can hurt monsters ordinary iron couldn't touch.
True Seeing: Useful. Sure the mage might have the spell as well, but you'll have to find the scroll for it first. Great for finding illusionary walls. Identifying cursed items is useful if the mage doesn't have the Remove Curse spell. If the mage doesn't have the Detect Invisible spell, True Seeing is a lifesaver against greater feyrs.
Improved Identify: VERY useful. Ranks up there with the cleric's Create Food. Without this you will be dependent on scrying mirrors, which are very rare and have limited uses, to identify any magical items you find. This is especially true for weapons and other items which can not be identified through trial and error. Unfortunately, with a tripple multiclass you do not start with the spell and have to hope to find the scroll early in the game.
Various attack spells: Sometimes Usefull. Between the cleric's Spiritual Hammer and the fighters melee weapons, you won't have too much need for these. A few monsters it is good to kill quickly though. Fireballs are very nice against trolls.
The most useful attack spell I can't remember the name of (something like Mordekanen's Sword). Its a level 7 spell so don't hope on using it unless you're playing a very deep dungeon. The spell conjures up a magic sword which as far as I can tell is the most powerful weapon in the game. Its the only thing I can think of besides the wand of striking +3 that can actually deal a lot of damage to the Elemental Lord.