That is easy: Kuo-Toans.
They are nastier than drow and very alien all in a low CR package. Anyone who has fought pincer-staff armed, lightning-flinging kuo-toan fighter-priests knows what I'm talking about. Oh and don't forget the flypaper shields and dreaded kuo-toan monitors.
They are immune to a lot of magical and mundane effects (grappling, webs, lightning, many charms and illusions) and they can see invisible, astral and ethereal creatures as an extraordinary ability. They also are one of the few aquatic races who suffer no penalty for operating on land.
Aboleth are cool too but not nearly as flexible.
Tzarevitch