Being a bit biased my vote goes to Corine Dorland. Also because of how long and succesful her career was in a time when professional female riding simply did not exist.
But really, the current generation are pushing female BMX forward so much it is very hard to compare them to the old on same footing.
I would say the women that were the engine behind this should be recognized for this. This is the whole USA and French top of female riders, each and every one of them, and also riders like Buchanan, Reade and Walker worked very hard to get where they are.
Ask me this again in 4 years time, when the major steps female BMX is doing at this time have settled in. Right now, it's just impossible to compare.