This is the way I see it. The knights of the old republic wielded acient powers that were way more powerful than most new star wars characters. I know little about most of these characters except for the ones in KOTR.
Here's my list of most powerful in no order. (there are otehrs who should be on this list though)
Revan
The Exile
Malak
Palpatine
Yoda
Luke Skywalker
Darth Traya
Mace Windu
Darth Sion
Darth Nulis (or however you spell it)
Darth Vader
Well Revan beat Malak so that gets rid of him and Kreia stated that the exile was the greatest she ever trained and he had far more powers. So that gets rid of revan. Darth Sion can't die so he is up there but the exile still managed to outpower him and sap his will. So Sions out. Darth Nulis was defeated while the Exiles power was still growing but that fact that he can consume all life on a planet is rather impressive still but he is out. Traya was beaten by the exile. Palpatine seemed a little about Yoda. Vader is but Palpatiens slave so he is out as well.
This leaves us with Mace Windu, The Exile ,Palaptine and Luke. First off I would like to say that all these characters seemed more pwoerful in the book (except for the exile). Mace Windu's lightsaber form is simply amazing. We never really see Mace use all his powers because they borderline the darkside. Mace Windus lightsaber form can be so fast that it is invisible to the human eye and that's why it's called vapaad. There is no lightsaber form that can really comapre to it in terms of lightsaber skills. It is said to be by far the most powerful form. It can even take the powers of another foe and relfect them upon them and draw from other foes power. It can find their weaknesses as well. Mace Windu reflected Palaptines own hatred and speed back at him and used at as his own. Mace Windu most likly has all the buffs of the lightside and the rage of the darkside to aid his physicle attributes but the Exile can get some awesome crystals and force enlightment and force valor which can make him pretty damn powerful. But Mace Windu wields the rarest and most powerful crystal. Mace Windu wins if the exile is a weapons master for completing the exiles juyo form. If the exile was a sith lord it would be different though. Palpatine can make a powerful ightning storm but since the exile has shown great lightning powers he probably could to but that attack is NEVER used for close range which is why he never used it. But having force storm when even Palaptine hasn't show to have it shows the exile was mroe powerful and he can sue force crush over and over again and decimate his foes. Luke passed Palpatine up in power so Palpatines out. We have the Exile (sith lord), Mace Windu and Luke left. At the end lukes lightsaber looked as if it were many moving at once but it was not invisible and he did not have the power of vapaad and he did not have the rare purple crystal (which in the books it was said only the most pwoerful jedi could weild it). I am just kind of rushing through this so excuse my grammer. Anyway Luke however could shoot emerald lighting in his last book. This attack would instintly kill a foe. It is unknown weather or not it is a lightside power only though. The exile might be able to harm him with choke or normal lighting or force crush (which is just as deadly for a single foe as lukes lightning). But luke has minipulated enitre planets and black holes. Luke is the son of the chosen one. However the exile was never really taught like a normal jedi and only had minutes to pick up on some things. The exile grew stronger for every foe he killed and learned while fighting his foes. I say the exile would learn vapaad and master it quickly and he would learn lukes powers. So the exile wins. Mace would beat luke in a lightsaber duel just for the record but not in an all out fight.