Personal picks, based of course on my own experiences.
Best:
1. Cone of Cold. Crowd Control and damage in one spell I can have by Level 2? I'll take that, thank you.
2. Crushing Prison. Slap it on any Lieutenant or Normal rank target and let it (almost) die while you deal with the rest of the fight. Being able to occasionally shatter frozen targets of even Lieutenant rank helps its case too, though the reduced duration hurts.
3. Mana Clash. Highly, highly, highly specialized, and requires the mediocre Mana Drain and Mana Cleanse to get, but is simply amazing when you do get to use it. With full mana, only Boss rank monsters can survive a Mana Clash, and even then you've just crippled their spellcasting. Of course, if you maximize spellpower, even the Bosses won't survive it.
4. Fireball. Area of effect + knockdown + damage over time + instant cast. Need I say more?
5. Force Field. Save the tank who's about to die, or cast an area of effect spell with impunity.
Honorable Mention: Haste. The attack speed acceleration is well worth the penalty to attack, and it even speeds up travel at that! At least one mage in the party should always have this.
Worst:
5. Telekinetic Weapons. Only very, very occasionally have I seen the increased armor penetration do any good, and even then, damage variance could explain the difference in numbers too.
4. Mana Cleanse. There are enough ways to kill enemy mages outright that Mana Cleanse isn't worth it. And if you're using it just to keep a mage pinned while your mage-killer talents cool down, you'll just end up making your Mana Clash that much weaker.
3. Heroic _____. Needs a lot of micro-management to make effective, and isn't that good to begin with. Only really notable as a stepping stone to Haste, though an Arcane Warrior can make decent use of Heroic Offense as a self-buff.
2. Spell Shield. 75% magic immunity is great! Except that it affects friendly spells as well, eats away at your mana (and collapses when you run out), and you ought to be killing enemy mages on sight, not trying to tank their spells.
1. Shapeshifter forms. So much potential, but crippled by the casting time and the mage class itself.
Honorable Mention: Cleansing Aura. I wanted to love that spell. But the mana drain and collapse not mentioned in the spell description as well as the caster being unaffected by it prevents me from really ever using it in battle. Saves on Injury Kits though!