My guess would be "Elf Boots"
The spell that makes your wizard able to escape any conflict.
When the enemy creatures are on a distance
I always think that I wont have any use for a pair of Elf Boots,
so I usually discard elf boots from my spellbook,
and ALWAYS regret it later on.
That is exactly what happened in my last game.
First I cast a druid, because I like to have magicians in my party.
2 of the 3 cpu wizards that was on a team against me
cast "Double" which means their next spells will be cast twice.
and they cast some horses, ghouls
and 2 of an other creature that they immediately cloaked
so that I didnt see what they were.
I had cast a grizzly bear and a king cobra by now,
and had discarded any "Elf Boots" spell as usual,
and these were even WINGED elf boots.
Then I cast a pool of life in my 'backyard'
so that I could boost my creatures lifeforces
to prepare them against the horde of enemy creatures coming towards us.
And that went well for many turns
I noticed that I had as many as 3 replicate spells
in my spellbook,
and when a "Sword of Sassenrath" spell became available to me,
I made a plan which was succesfull.
I Cast "Sword of Sassenrath" on my druid
who now had a high lifeforce.
And now with this spell, he had a good fighting ability as well
and could attack undeads.
I then replicated him 3 times
so that I now had 4 strong and skilled druids
who begun casting lightning bolts on the enemy.
And I used them to attack the undead creatures
that was queing to kill us all.
One of the enemy's cloaked creatures
attacked my wizard for several turns,
so I cast "Ball Lightning" on it to harm its combat ability
so that it was less of a threat.
Then, out of the blue, when things were looking good for me,
an enemy rhinocerous advanced right onto my wizard
from an unguarded path.
I had not cast any defensive spells on my wizard yet
(like a pair of elf boots)
So my wizard was stuck trying to fight the rhino.
Rhinos are very powerfull creatures
so my unboosted wizard had no chance of killing it.
The 2nd most important thing in Total Chaos is to protect your wizard.
without your wizard, you will lose everything,
So I cast a heal spell on my wounded wizard
to buy some time. (I saw no other choice)
3 of my druids was busy elsewhere, so couldnt help me,
and 1 boosted druid is of little help against a rhino.
On my last turn alive I cast "Recovery Boost" on my poor wizard
but a ghoul helped the rhino kill me.
If only I had a pair of elf boots on my wizard's feet,
I could escape the rhino and attack it with all my strong druids.
Do you think any other spell is more underevaluated than Elf Boots?