IMO the two most useful skills that aren't all that obvious are the warrior craftsman sharpen skill and the shaman nature hammer skill.
If you buy sharpen as soon as you get to the first battleground (early in chapter 1 unless you overlook the key or the chest), raise it to the maximum (unlearning other skills if needed) and sharpen a weapon needing no agility you will enormously increase your damage dealing potential as long as you have agility of 13 or so. You can then unlearn sharpen. The first BG dungeon provides plenty of cash if that is an issue.
If you get the hammer spell early in chapter 2 (again, at the BG) you really need no other magic for the rest of the game though a wide area effect spell is sometimes useful to have also. It does better damage than the description indicates, hurts any enemies near where it falls (often killing them outright) and usually paralyzes them for a few rounds. Through the final battle it is enormously useful - even a party of two warriors benefits from being able to cast it even at low power levels. My warrior with intelligence of 1 hastily unlearned other skills and learned hammer in the middle of the final battle my first game and won an otherwise hopeless fight as a result.
