25-3 is the best place that I have found. It gives 907BT per successful run. (More, if you are using one of the Christmas units.)
You may want to double-check your BT point totals tho. I've been reading reports that the quests from chapters 1-4 (all the quests which have a cooldown of less than 5 minutes) do not give any BT points.
I have not personally checked the BT point gains from 2-7, so I may be wrong.