Steven Fright is "best", he will be the one who is most effective at everything. He is.. put simply, the hardest AI that V0id can come up with. The others probably wont be a "fastest" or a "slowest" but might be instead a case of Underlord X prefers beasts and wrath aspects, just as an example this could be Draven or Rhaskos (again i'm speculating.. nothing im saying here is fact)..
Whilst Underlord Y prefers turtling with Sloth aspects and building up a large army before attacking you, etc (perhaps this would be Oberon?). They'll all be "best" at their own chosen playstyle.. and Steven Fright will be even better.. at everything ever.
Also there is a bit of talk about difficulty settings for the AI too, from 1-5 Difficulty settings depending on how badly you want Steven Fright to kick your ass.