To answer which dungeons are required up to lvl 50 and which ones are optional:
Lvl15 Sastasha (story dungeon for quest "It's Probably Pirates.")
Lvl16 Tam-Tara Deepcroft (story quest "Fire in the Gloom.")
Lvl17 Copperbell Mines (story quest "Into a Copper Hell" )
Lvl20 Halatali (OPTIONAL)
Lvl24 The Thousand Maws of Toto-Rak(Story "Into the Beast's Maw")
Lvl28 Haukke Manor (Story "Skeletons in Her Closet,")
Lvl32 Braflox Longstop (Story "The Things We Do for Cheese.")
Lvl35 Sunken Temple of Qarn(OPTIONAL)
Lvl38 Cutter's Cry(OPTIONAL)
Lvl41 Stone Virgil (Story "In Pursuit of the Past")
Lvl44 Dzemael Darkhold (Optional but needed for some Grand Companies)
Lvl47 Aurum Vale (Optional but needed for some Grand Companies)
Lvl50 Castrum Meridianum (story)
Lvl50 Praetorium (story)
Lvl50 Wanderer's Palace (optional)
Lvl50 Amdapor Keep (optional, but required for your relic weapon)