Olympus and Tartarus. Olympus because it's easy and gives great gear for lower level players. Tartarus because it's a fun challenge that you can do solo (long haul fight) or with friends for a quick a run and great gear and reagent drops. The secret bosses in each make for nice short runs that you can do alone or with friends. They are both perfect difficulty level for their respective levels. Atlantea is the one Aquila dungeon I really don't like much. The drops are pretty pointless at that level, time would be better spent in Waterworks and the secret boss is just a pain with nothing really good to drop, unless you only want a pet.