The agent is considered the second best, almost a close tie. And it actually expects you to be human and an operative. First all Imps in power are Human and secondly, infiltration isn't really a Sniper's forte. However, at least the story line makes it apparent that an Alien will have it hard in Imp. Sec. Unlike the Sith stories in which it's glossed over and sometimes assumes that you are Sith born, despite being a Human or other race.
The BH is the 'best' (by an INCREDIBLY small margin) because it makes no assumptions and works, no matter what you are, male, female, human, alien, powertech or merc.