For Empire the most underplayed is probably Sniper or Powertech.
Operatives were originally the least played, but when people found out how powerful they were, everyone decided to roll one.
Indeed. I did a WZ last night on my Sorc Alt (level 21) and there were no less than 8 concealment Operatives and 5 mercs there. Given that I was the only healer, it was constant faceplanting and tracer missile explosions. I spent half of the time face down in the dirt, the other half in the rez area.