Laidbackgamer: -Paralyse: temporary flesh-to-stone spell. Lasts quite a while, even at lower skill levels has a good chance of working, and if the enemies affected by it are the only ones standing, they count as dead for the combat system purposes. Powerful monsters ("bosses") will more than likely be immune to it, but it's a great crowd-control spell
-Visibili: makes the party invisible. It's more of a utility spell since it allows a player to bypass most random encounters while in a town (although the plot will always find you, so important "random" encounters will ignore the spell). Comes in handy in RoA3, from what I've read.
In the Combat school there's:
-Fulminictus: direct-damage spell. Pretty good all around.
-Ignifaxus: direct-damage spell. It can be boosted when cast. I might be wrong, but each level of boost should equal to an additional 1d6 in damage.
-Plumbumbarum: debuff spell that reduces the attack value of the target. Lasts the entire combat and isn't too expensive to cast, so it's pretty good.
These are the spells I've learned are useful in combat (except Visibili, but it's a Transformation spell, so I've included it). I actually have not played that much, so I have not had a chance to test out the other spell, especially those from the Domination and Dispell schools, so if someone more experienced in the game can chime in it would be great. But the spells above-mentioned did prove valuable in a few difficult fights I faced.
although not quite apparent in the first game, dispell domination/transformation are quite useful in the latter two games as the appearance of monsters that can dominate you or turn you guys into stone is a lot more common.
the following spells in the domination tree are ones that i have personally found pretty useful
-Ball and Chain: if you're outnumbered and your fighters cant keep the enemies to themselves, endangering your casters, you can use this to immobilize an opponent and pelt him with spells/ranged weapons.
-Somnigravis: effective crowd control. like lightning, it stops your opponent from performing actions.
-Horriphobus: causes enemies to flee, helps end fights quicker and you still get exp for it.
Combat-Terror Power is a great melee buff for your fighters, i recommend getting that as high as you can.
And dont forget to cast odem arcanum/analyze on things you suspect are magical!