Depends on your definition of a primate.......
The Anglican Church has Primates....The new Primate for Australia, Archbishop Peter Carnley might be a good candidate for the peddler on the rear seat of a Tandem - No whip required, You could keep his spirits up by singing Onward Christian Soldiers at the top of your voice and feeding him Communion Wine when his spirits appeared to be flagging.
Excluding Humans - A Gorilla would provide the most power, but might not like the road you wanted to travel along - In which case you better have something slightly more substantial than a horse whip to hand if you are trying to get him to change his mind.
All in all - I would stick with the Anglican Primate.