For pure stunting I would say, wr250x cause its light. and you can get some stunt parts for it like rear pegs, handbrake, sliders etc.
but... I like my ktm 450exc,  I ride it on the street with sm wheels,  then put dirt wheels on for the weekends.  Tried a little stunting, I can wheelie no problem, some short stoppies,  got a 58t rear sprocket to try circles but I kinda gave up on that.