Solo or with a group? Assuming solo. Freeport Militia, Lower Guk undead side, Tofs though the 6th floor is mostly Lb to me at 52, Gunthak harbor, Mistmoor in a corner room area near the garden is nice well. Im looking around for newer areas myself.
at 51 i was soloing the graveyard in Katta or Duo'ing the Vamps outside with my wifes Necro, other choises are echo caverns Taskmaster camp, PoInnovation groups are good Xp in the low 50's and will teach you valuable lessons for tier 2 planes. LDoN of course Rock Hoppers in DSP if you want PP or head to LoY and dulak harbour either a ship or the small harbour/town in Gunthak before you get there.