The Right Side of Ludibrium Clock Tower
This place provides some of the best training for level 70+. Note that the levels I indicate are nearly useless; if you have a DK, the only thing that matters is the amount of HP you have, which will vary from person to person based on the amount of HP gear. Also, when I say party, I usually mean a party of a priest and DK, and nothing more.
The place you should train is based mainly on the priest's survivability. Clearly if you can't survive 1 hit, you can't train there. The second factor depends on the DK's level. If the DK is level 70-80, parties at DT are the best. From 80 to early 9X, MDT are the best. From mid 9X onwards, Grims will become the best training. But those are just general cases.