Weightbelt under the crotch strap is my choice.
I have a short 21in torso, so my plate rides just above my hip bones. My weightbelt rides just a few centimeters below the waist strap.
For other people who are short or have a short torso, it is impossible to ride the belt over the crotchstrap, because it will have to wrap either around your buttcheeks or over the plate (and believe me it's not secure either way). That or you have to hitch your plate up real high which puts the tank valve above your head.
Some opt to go with a harness, which I use when I'm assisting classes (on the instructor's orders).
The DUI classic harness I use has it's pockets coming just at my mid thigh, nearly blocking my wetsuit pockets from opening, and worse creating a bit of a block when I have to squat down.
It also puts all the weight on my shoulders, which the Backplate already does. The result is wearing all that 50+ pounds of tank, weight, Bp and all on your shoulders. Makes me sore as hell after a day of diving, so I'm not a huge fan.