Hotspots are generally Blackburrow / Crushbone / Kurns for the first bit, depending on your continent.
At 15+ (or 20+), these change to WK bandits or SK aviaks / Unrest or MM / Overthere or LOIO. Highpass Hold is also an option 15- low 20s. Upper Guk is also amazing xp if you don't mind ruining live froggy faction.
At 25+, Highpass Keep is probably one of the faster xp spots to 30 or even 35. MM continues to be good xp at 30+ too, and Sol A isn't bad either. 35+ Lguk opens up as viable, as does Sol B entrance.
40+ is CoM to 50+, then Seb, Velk's, and several other spots open, as well as deeper into Lguk / Sol B.
Think that covers most of the spots
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