I think Syrcus Tower is the best time-to-tomestones ratio content to run, but if you prefer dungeons, probably Tam-Tara, since it just requires burning bosses and mobs down for the most part. Copperbell has the weird gated fights, Toto-Rak is infamously long to run thanks to the sticky flooring, and Qarn and Vigil can be tricky to run if you aren't careful. This is presuming you're running via Duty Finder and not with a pre-formed group, in which case it would be whatever you prefer.