Very tough cause it depends if you go by George Lucas ideas or what happens later in the EU or even what happens in TOR.
I recall Lucas in a commentary (please don't hate me if it was an interview) saying that Ankin/Vader became weaker in the force cause he was part machine. As we all know Machines have no connection to the force, you then have the question about how midichlorians effect someones force powers. If Ankin had more than Yoda was he more powerful or is it knowledge that makes one more powerful in the force.
Then of course you have the question about where stupid things happen in the EU or games because x y or z is the hero. The Force Unleashed, along with pulling a star destroyer out of orbid, beating Vader and then if you play the what if beating Luke after turning him dark.
If Canon is 6 films and the Clone wars my Money is on Darkside Anakin right afer he chops up Younglings. If its all the EU then Exar Kun must be up there given he had an artifact that multiplied his powers was it 1000 times, been a while since i read the comic. But then you have the TOR Emporer who