[Legolas]

The sorcerer class would definately be the hardest to use effectively, because you can only learn a limited amount of spells. That means you need to understand how all the spells work in relation to one another and in different situations, or you'll be wasting potential.

Between the cleric and the wizard, the cleric would be the most forgiving. It lacks some of the raw damage wizard spells can dish out, but in return you get a very versatile class. Spells like Find Traps and (if you take the right domain) Knock, are all you need to replace a rogue or pixie familiar. The trickery domain offers invisibility spells if you're interested in that kind of thing, which can further maximise your efficiency in that field. Especially at higher levels, or with the aid of the Magic and Air domains, you have access to magic capable of dealing damage or killing outright. Healing spells are a bonus, and so are the reverses such as Harm. Undead will hate the cleric no matter what.
More importantly perhaps, as opposed to the other two classes, the cleric is not bad as a fighter. In fact, with the right spells he can temporarily become a better warrior than any pure fighter of the same level. You can also cast those spells in armor.
Finally, you do not need to limit yourself to just a few spells, and you do not need to hunt for scrolls. You know every spell in the book from the onset.
It's hard to go wrong with this one.

The wizard and sorcerer, however, do have more killing power in terms of magic. Theirs is the pure spellcasting class, and if you're interested in abandoning weapons entirely those are the classes you should try out. And as I said at the start, the wizard would be the better choice for a beginner here.