James Bond vs Darth Vader
I watched "Portrait of Brenda", and realized that this episode is as close to the title as you will ever get, Roger "The Saint"/"James Bond" Moore fighting David "Vader" Prowse. Of course Prowse doesn't wear the Vader mask (it hadn't been designed yet) and has his own voice, but... in any case, an episode not to miss for Bond and Star Wars fans!
Actually, The Saint has lots of Bond stars in various episodes (some more even in this one), many who were in Bond movies long before Moore, so David Prowse is the big deal here, fighting Moore as much as twice.