Foresight gets my vote. A 9th level spell that gives you a +2 to Reflex saves and AC. Give me a break: the 4th level Divine Agility gives you rogue Reflex saves, Spring Attack and 18 Dex.
Other than that, there are a fair few.
The lowdown (one from each level):
1st:
Detect Undead (ooh, look- a skeleton!). Sure, you can use it to detect vampires, liches and the like, but it's a pale shadow on Detect Evil.
2nd:
Tasha's: Rather sad as the only Enchantment available; compared to Glitterdust or the clerical Hold Person, it's pretty weak.
3rd:
None: all of them have their uses, even the infamous Gust of Wind (which can get rid of cloud spells).
4th:
Shout: A puny amount of damage as a 4th level spell, and the deafness is no big deal either (compared to e.g. Confusion or even 2nd level Glitterdust).
5th:
False Vision: Probably coming high in the 'least known spell' category, it's fairly puny. The 1min/level duration means that unless you know that the evil wizard will scry on you at 10:30am, you can't really use it at all.
6th:
Analyse Dweomer: Of course it's useful, but should it really be at higher level than teleport, raise dead and permanency?
7th:
Insanity: Please name me *one* instance where you wouldn't be better serviced with Feeblemind, Hold Monster or Dominate Person...
8th:
Iron Body: Have you ever seen it used? Me neither.
9th:
The infamous Foresight, the world's most underpowered spell for its level.TM
