The best spawn will depend on some factors, like your level, skills, etc.
Assuming that you are leveled enough to get any full respawns without trouble, Sunken quarter would be the best spot (since it has a lot of quaras in such small spawn and you can divide which quaras you want to hunt according to the floor you are).
Calassa has the advantage of having a small chance of finding Quara's boss, Thul (doesn't drop any rar valuable item, so it would be just the satisfaction of findind a rare boss).
There is also the spawn of Frozen Trench (located on Svargrond, you'll need to make the quest for its access, though), that even though it is not a great respawn, for low levels it may be the best one, since there is a lot of free space to avoid luring a lot of quaras altogether.