[Philos]

I agree, THE bottom 2 have to be (IMHO) potion of infravision and potion of insight (enhance wisdom). Every area of IWD is lit so who needs infravision. The insight potion only lasts 6 hours so won't buff cleric/druid for any increase in spells taken and there are no saves directly tied to wisdom in IWD I am aware of (Insight is VERY useful in BG2 for mages though).

I disagree about potion of genius however. Even a mage with 18 intelligence has a 15% chance of failing when copying a spell to their spellbook. Using the potion of genius bumps their Int to 23 (I think) and lowers that chance to 1 or 2%. Unless you have your game set on easy, which disables the chance of spell copying failure, that is a pretty significant help. On my first run through I had a mage (with 18 int) blow copying a fairly high level spell which you usually only find one copy of (don't remember which one now). From that point on I save scrolls and copy a bunch at once after taking a potion of genius. Haven't lost one since in three or four subsequent run throughs.
I am not sure of this next reason for IWD, but it is true in BG2 and so it might be true here. The potion of genius also allows you to copy more spells to your spellbook than the limit normally allowed by your intelligence. Both games use 2nd edition rules and an 18 intelligence is limited to something like 17/18 spells known per level. That is still a lot but potion of genius lets exceed it.