PetrusOctavianus: The Fighter/Mage is potentially the most powerful characer in your party, if buffed with things like Mirror Image and Globes of Protection.
The magic user is rather a specialized character since in some battles he plays hardly any part, while in others he Fireballs most of the enemies. That's normal.
Personally I often end up using the mage to set up backstabs for the thief.
As for which spells are best...
Lv 1:
Sleep is very useful for lvl 1 characters facing stuff like Orc, Goblins and Kobolds.
Enlarge is very powerful once your mage reached high levels. It significanly increased your party's total damage output if cast on all your Fighter types. Usefulness is dependent of playing style, though. Not very useful is walking around with Search on all the time, but if not it can last several hours of in game time.
Charm Person is potentially very powerful if you can get a high level humnoid to switch sides in combat.
Magic Missile is good for disprupting enemy spell casters of to damage especially well armoured enemies.
Lvl 2.
Stinking Cloud is a fabulous spell that even Dragons often fall victim to.
Other lvl 2 spells are generally not very useful.
Mirror Image is a good buffing spell in the most difficult battles.
Knock is sometimes neede to unlock doors.
3.
Fireball
Lightning Bolt
Slow (can be useful against groups of enemies with high HP)
Haste (very good buffing spell before dificult battles)
One Dispel Magic can also be nice to have in emergencies.
4.
Ice Storm is nice. No saving throws.
Charm Monster rarely works, and only targets two enemies.
Minor Globe can be nice.
Fear is generally useless, and it also affects allies caught in the AOE.
Confusion can be useful against large groups of enemies, but in practice is rarely very effective.
5.
Hold Monster can be very useful.
Cloudkill can instantly kill low level enemies. Also good for disprupting the spell casting of groups of mages that are high enough level to not be outright killed, by harming them each round.
Cone of Cold: hard to target, but can be very effective.
Thanks again Petrus for all the great tips. I'm having more fun figuring out and playing these old games than pretty much anything out today. They've got a vibe and a charm that is just amazing. Sounds like I should be using my fighter/mage in melee. All this time I've been having him use a bow. I will try that out along with protective spells and let my single class mage do all the pew pewing.