Sennisten is the best. Lots of temples, a palace fit for a God, lots of homes and shops and a grand ol' slum. We even got an Insane Asylum. Our streets are also the cleanest in Gielinor thanks to Chthonian 'n' Co. Street Sweepers.
For me a tie between the best places to roleplay are the BTS Bar, and Darkmeyer. While the prior is not a city, it is absolutely massive, with plenty of space for everyone to get along. As for Darkmeyer, while it might not be easily accessible, it is, again, huge, and just edgy enough for me to absolutely love it. Complete with dark blue lighting to make the blood stand out."I am true divinity! I am the father of nations and peoples! I am the thing you merely pretend to be! I am God, and I am King, and you can never defeat me!" ~Galath, Infinity Blade