It depends on what your other invocations are. Spiderwalk is an excellent least invocation and in many dungeons is almost as good as flight, but if you actually have Fell Flight, not much reason to have both.
Other fun least invocations:
Baleful Utterance: Awesomeness already explained above.
Call of the Beast: Not very powerful, but being Dr. Dolittle can be useful in the right campaign (gives Wild Empathy AND ablity to communicate with animals)
Summon Swarm: Very useful to have at will, and the fact the duration is "concentration" instead of "concentration +2 rounds" as the spell is actually a boon. If the swarm starts attacking the party, just stop concentrating and it goes away.
I also like Eldritch Glaive and Entropic Warding, mentioned above. Darkness + Devil's Sight is great in tandem, nearly useless individually.
For essences, Sickened and Shaken do the about the same mechanically. More enemies will have low will saves than fort, but more enemies will be immune to fear than sickened, so they're about even in usefulness.