It really just comes down to personal preference. I finished Risen as a mage with level 9 Frost and level 6-7 Fireball.

Once frost gets high enough, it does a fair amount of damage and also freezes your opponent for a short time, during which you can pound on them with melee weapons.

Just remember that you won't have anough learning points to develop all of them. Pick a main one to concentrate on, and then you can add points to a second one later if you want.
 
Bullet is the fastest and most accurate attack spell but deals moderate damage. The main advantage is however the speed. There is no recharge animation either, so you can disable foes in seconds while standing infront of them even in close range. Each bullet hit also staggers the attacking enemy. If you go this route make sure you have enough mana :)

Fire deals the most damage and has a nice area effect. When fully upgraded 2 or 3 fireballs is enough to kill even the strongest foes. The downside is that recharging fireball takes time, so its best to attack from long range. This gets easier though when spell becomes better.

Ice deals least damage, but it can freeze enemies. Then again you need to finish the frozen enemies with other means.

For a warrior mage ice or fire is likely the best choise. If you go pure mage route magic bullet or fireball is the best. My mage character had a fully upgrade magic bullet and an huge mana pool. I was quite unstoppable. I also did some staff and strenght training:)
 
I played as hardcore mage with all three spells. At the end it came down to bullet and a rather broken and boring spell system. Fireball is supposed to deal area damage but this is meaningless since what you wan't is throwback (meaning that spells block foes running up to you). The only drawback to bullet is the manacost which is meaningless at higher levels since you will have so many potions anyway.
 
I prefer ice when playing a "battle mage" (for example Inquisitor) and fire when playing a pure mage. Fire deals a lot of damage and feels very mage like, while bullets feel more like a machine gun.