whats the best one for a spear?

Well for spear, you should be planning on going pure int (or close to). In which case, lightning is a must. It's a better idea though, to raise both fire and lightning. Use the fire imbue, thats better than the lightning one. At lvl 30 fire and lightning, you can get the 'nukes', which will be your main attacking skill from then on.

Take a look at the "Full Int Question" thread on this forum.