For Imps I'd say:
Bounty Hunter: Skadge
I significantly doubt Andronikos. Lots of assassins who want a dps companion who is darkly aligned will use him.
Skadge and BM are true, but that's partly because they're acquired late in game. They're both darkside tanks who get sick affection rewards in conversations, so people who can use it should consider using them.
Knight: Sgt Rusk, darkside companion (not popular with Knights) who is acquired at pretty much the same time as Lord Scourge, who is lots cooler.
Consular: Iresso. Who finds this guy interesting anyway?
Smuggler: A hard one. I'd say Akaavi the least, but that's merely a guess.
Trooper: Yuun or Tanno. (Yuun because he's both lategame and melee dps (and because he's so easy to sway with gifts), Tanno because he's DS, and apparently people don't realize just yet how bad *** he actually is).