Breach:

Er... only because everybody does? Seriously though, Breach is one of the MUST HAVE spells, it removes Stoneskins and other hindering spell protections. Without Breach, your fighters will have a very hard time defeating magical opponents.

Lower Resistance:

If you want to survive the game properly, you will need this spell... trust me. This spell enables you to use your magic against highly magic resistant enemies. This is a good spell in Dragon fights, and its' value will be highly appreciated in ToB, where there are many many magic resistant enemies.

Spell Immunity:

Ice Storm? Meteor? Spell Immunity ----> Evocation, Imprisonment? Spell Immunity ----> Abjuration, etc... ... This spell is a potent protection spell when used properly.

Animate Dead:

95% magic resistant, immuned to normal weaponary, lasts 8 hours, immuned to psionics, an average of 20 damage per round and good THACO, this is my favourite summon, try it, you will love them too.

Feeblemind:

When successful, the target becomes dumb and does nothing but stand still forever.

Hold Monster:

Some people prefer this spell to Feeblemind because it affects an area. However, many large animals are immuned to Hold Monster, Dragons cannot be held, they can be Feebleminded though.

Chaos:

Although it can be effective, it is highly unpredictable. I prefer casting spells knowing what will be the results.

Domination:

Usually resisted, this spell is not really useful since those that get dominated are usually weak in the first place and better dispatched with swords and such.

Cone of Cold:

Chances are, you end up hurting your party more than the enemy. True, you can position your mage to avoid hitting your men, but this wastes time. Be warned though, that Cone of Cold destroys loot if it is the killing blow.

Sunfire:

Like Cone of Cold, you may end up toasting your party members as well... ...

Cloudkill:

You want to use Cloudkill? Get a wand... use it until it reaches one charge then sell it to merchant and steal it back. It will now have 50 charges. When it drops to one charge, sell it to a blackmarket merchant and steal it back again. Proceed to use it until it runs out.