In my opinion, I'd say Luke, Leia and Darth Vader are relatively equal.
This is based on the fact how fast Luke learned to block the remote's stun blasts and used the force to -- well, force the torpedo into the exhaust port without a targeting computer.
Luke losing to Vader on Empire was because A: Vader had 30+ years of experience from the Phantom Menace, B: as a Sith, he doesn't hold back, C: Luke was very inexperienced, and let his arrogance get in the way like Anakin's did when he lost the battle -- and his arm -- to Count Dooku.
Luke also beat Vader on Jedi by almost falling into the trap Vader fell into years prior and unleashed his rage against him, and Vader's uncertainty and reluctance to harm his son.
And Vader based on how Qui-Gon Jinn sensed something in Anakin Skywalker and saw how his reflexes are during the pod race and the test he took by the council.
I included Leia in this due to bloodline -- aka Luke's twin sister.