Who would win?

Anakin (Darth Vader before being burnt ROTS) vs ROTJ Luke.
Is Anakin the strongest ever in ROTS when he turns?

Anakin would slaughter Luke from ROTJ. And yes, Anakin is in his prime (even though he had much potential to increase his power) after he turns to the dark side in ROTS.

Anakin would win. he has more training.

Anakin fully trained Jedi Knight with a shit load of potential and who actually knows how to use the force fully in combination with a lightsaber against an untrained farm boy like Luke....

Hmm let me see here, Anakin

Anikan