To me, the sewers are the best and you have two options:
The experience is good, on level 300 (Team) I get 6kk per hour.
The loot is not than god, but you can pay the waste.
You can face Black Sphinx Acolyte, Burning Gladiator and Priestess Of The Wild Sun.
The experience is lower than the first option, on level 300 (Team) I get 4.5kk per hour. Because the monster attack each other and this reduce your experience.
The loot is not than god, but you can pay the waste.
You can face Black Sphinx Acolyte, Burning Gladiator, Priestess Of The Wild Sun, Crypt Warden and Sphinx.
Also you are in the city so can exit and refill.