My favorite Outsider? Hmm ... I'd say Arsenal before the days of Red Arrow or the New 52 or any of that. There was just something about Roy Harper, trying to be a leader after having fallen so far and to work beside former teammates like Nightwing and Starfire who hadn't strayed from the heroic path ...
Huh? We're not talking about the early 2000's DC Comics title "The Outsiders"? What are we talking about then? Dungeons & Dragons?! Oh, crap!
my Games tended to feature either complete Tolkinish creatures or went more toward the Robert E. Howard and H.P. Lovecraft side of the spectrum with ancient unknowable things that required me to still the sanity rules from D20 Call of Cthulhu.
That said I've always enjoyed the Formians. Something about giant sentient ants that could use tool has always given me a shudder.
I was first introduced to the Formians in a Forgotten Realms novel that I forget the name of, but dealt with a legion of soldiers trapped in an old fort and try to defend against a force of invading Formians. It was a great read for no other reason the the comparisons the author kept making to the hive minded Formians and the soldiers of the legion. In game I only used them once, and then it was a one-shot Fourth Edition Dark Sun game that ... honestly was horrible quite horrible.
I would now lament my dislike of 4e, but I am not one for edition warring and honestly, I enjoyed the system greatly during my days as the D&D Encounters Dungeon Master for my old gaming store (before I moved).