Inferno, hands down.
And they had seemed so promising too...but it's true, the only worthwhile creature is the Succubus and her very fine upgrade, and their 2nd best creature is the Imp upgraded. I couldn't believe how largely useless the devils were, and don't get me started on Horned Demons!
I'm suprised Academy is in the running for 2nd place. I agree that they get far too much Knowledge, so much that you don't know what to do with spell points. By the end of a rather large map, my main Academy hero had over 20 knowledge, but only about 6 spell power (without artifacts). Sure, I can sling spells all day long, but it doesn't do much at all.
But with the most shooters in any faction, I thought the Academy was very strong...they're relatively quick too, which is devastating with Mass Slow or something. Kill everything before they have a chance to act? Count me in!
And the Necro debate...they rock!
Having well over 1000 skeleton archers drew fire like you wouldn't believe, and they really packed the damage. All you needed to do was Raise Dead on them near the end of the battle, and one cast got me back about 300 with Vladimir. Spectres are also great focus targets (seige towers miss them so often it's funny), but are easy to raise with their low hp. In fact, in one map, I easily conquered everything with my legion of archers, 100 ghosts, and 60-70 vampire lords (love those self-ressurecters). Putting in any other creatures spread damage more, so I had to Raise more stacks than the 1 or 2 that take the beatings.