Romulans, definately. There's really only a handful of episodes from each series that deal with Romulans, and there's never been in real development for them. Certainly not the the extent the Klingons have been developed. The only episodes that really examine Romulan culture or Romulan life are Balance of Terror, The Enterprise Incident, and Face of the Enemy. Add to that the've only been prominent in two movies. However in one, they played second fiddle to their newly introduced slave race, and in the other their homeworld got destroyed.
Voyager seemed to have a problem of creating cool aliens, and then never re-visiting them. The Vaadwuar were really cool aliens, and Dragon's Teeth even leaves the door open for them to return. The Hirogen were also really cool. And although we saw them more than once, they were rarely used. Voyager could have improved itself tenfold by developing these races instead of constantly running back to the Borg.