Hello !
Sorry for spamming you with threads about this game, but I'm hooked.

So I want to be a fighter/mage but there is a problem with magic.
most spells are useless.
For example in the Fire Magic you got 5 summoning spells but only 1 creature may summoned at time ,thus you will more likely choose and use only the strongest creature.

In earth magic there are 3 golems but you will only use the strongest one.
Also there are few protection spells but you will only use one.

What magic is the "best" in the game ? what spell does the most damage ? what summon is the best ? what type of damage is the most useful ?

You can get more than one creature at a time, just not the same creature(there are also cards for this), or maybe they can't be from the same school of magic, though creatures are best at being a distraction (which is hard to accomplish reliably) because they take damage far better than they dish it. Best use for them is to trick them into walking through a bandit camp for example to get everyone attacking them instead of you which is very useful, but If enemies see you first they are useless. Once you attack an enemy they will only ever attack you.

Damage spells are unfortunately worthless. Even if you boost and stack the best of them they aren't much use except for ice ray on the end boss. Far better are freezing spells like the ice wave in water magic, chains in earth magic, and ivy in earth magic. Ivy and the freezing wave are tied in my book for best spells. Most others look cooler than they are useful. Note the freeze spell will let enemies attack you one last time but is still obscenely powerful. Think freezing 20 people in a cluster and dropping a hell bomb at their feet. Instant level up with no sword.

The only other decent spells are Earth magic's protection spells that let you take more damage. Some others might be useful if you had help, but playing though the game you are on your own so.... However, do note that I have never used necro spells so I can't speak for them.

I tried playing through as a pure mage and gave up on it half way through. Robes are expensive for what they provide and the best staffs arrive early in the game so you stop realizing any potential early on. In short its hard, expensive, frustrating, and unrewarding.

I tried as a mage. I'd advise you to take my advice from the second to last post of this thread:
http://www.gog.com/en/forum/two_worlds/anyone_got_any_tips_for_playing_as_a_mage