Type: Imbuement
Strategy: Giving the caster 18/00 strength, an increase to THAC0, and additional hit points, holy power will help your priest act a bit more like a fighter. If you already have high strength, though, the main use of this spell has been negated. So, if you need the help, use it. Otherwise, go for a spell that will bestow a few more bonuses.
Importance: Low
Type: Cure, heal
Strategy: The wonder spell for vampire hunting, lesser restoration will annul most energy-drain effects like drain level. Since these are extremely common when hunting vampires, and hunting vampires is itself very common, a copy of this should be memorized at all times.
Importance: High
Type: Charm
Strategy: This spell lets the caster gain control of any creature that fails its saving throw. Otherwise, the spell acts strategically much like any other charm spell, such as charm person or mammal. In this case, though, the difference is that any creature can be charmed. Also, this spell is resisted much less and works incredibly well against most low- to mid-level creatures.
Importance: Moderate
Type: Defense
Strategy: Bestowing partial protection to undead and spells like drain level, negative plane protection only works when fighting creatures like vampires. Though the though of never worrying about drain level is nice, this spell only affects one creature. Since vampires have a nasty habit of attacking everyone, it serves little purpose.
Importance: Low
Type: Cure, heal
Strategy: In Baldur's Gate II, most poisons will require a potent spell to neutralize them, and this spell is your best bet. Not only will it dispel the poison, it will heal a few hit points. It will even cure blindness and deafness. It's a four-in-one spell and comes in handy all the time.
Importance: High
Type: Missile damage
Strategy: Dealing continuous damage that gets more potent and has a longer duration as the caster levels up, poison packs a good punch when it is cast successfully. However, poison seems to be one of the easiest spells to resist. So, unless you use a magic resistance decreasing spell, poison can't do anything at all. If you do use it, try to find a very potent damage-over-time spell that is great to start a battle with, then follow it up with other damaging spells.
Importance: Moderate
Type: Defense
Strategy: Like a normal protection from evil spell, this spell will become increasingly more useful. The difference between the two is that this one's range is ten feet instead of one. Not only is it a must for summoning minions, it will protect you against many demons found in the later parts of the game. One particular battle comes to mind where there are hordes of demons, and casting this spell will let you turn the tide of battle from an impossible situation to an easy victory. However, if you happen to have a class like a paladin in your party, he will come with plenty of normal protection from evil spells as his special ability. So, if you value the range boost, this spell is worth it. Otherwise, stick to the normal paladin ability.
Importance: Low or high, depending on party makeup
Type: Defense
Strategy: Working like all other protection spells, protection from lightning confers immunity to all electric damage. It's useful if you are up against a mage who likes lightning bolt, or if you find yourself in an area with lightning traps, but usually, it's is of no help at all.
Importance: Low