Cat's Grace (lvl 2 arcane) +1d8 dexterity, max 20
Why? Because if you have a dwarf or 2 in your party, it is a good buff. That dwarf warrior with 17dex now gets +1 armor class for 1 hour/level of the caster. That's pretty good. The dwarf fighter/thief you took with you for traps and locks? He gets the AC bonus, and his thief skills are better. This spell is useful throughout the game.
Friends (lvl 1 arcane) +5 Cha for 3 rounds + 1 round/level
Why? Because you can't get reputation increases in icewind dale. Have your Cha18 bard cast this, and suddenly your 15% discount jumps up to 25%. Very much worth a lvl1 spell slot. Baldur's Gate
Strength (lvl 2 arcane) sets strength to 18/50 for 1 turn/level
Why? How many NPCs have Str under 18/50 who could really use that buff?
Haer Dalis (17)
Mazzy (15...though she can self buff)
For most of the game, the best you can do is buy a belt of hill giant strength, giving one of these characters Str19. For the rest? This spell really does help them. Even characters like Nalia, Imoen, Aerie...if they are using slings or throwing daggers for when you want to save spells, then Str 18/50 will help them do more damage.
Up until ToB this spell remains highly useful. I can't get enough of it.
So...which spells can you just not go without, with the exception of stables like stoneskin and magic missile?