Assaj...
Luke win because Vader was just douch moch him.. The problem of douch moch is tha the target is to convert and not kill so you are more expose, and even more if that's successfuly happen because then the Jedi using Dark force power kicked your ass....
Assaj will just salughter Luke without a question..
ROTJ Luke's combat prowess is really difficult to measure considering he only fought his own father who was holding back on him.
I'd say he's anywhere from TPM Obi-Wan's level (as a minimum imo) to Late CW Ventress's level (best case scenario imho).
So I don't see him beating Ventress realistically. He may match her though if he's really that good. But he's not doing better than that.