I have watched all of the movies multiple times and read almost every single EU Star Wars book. From all of my amassed knowledge, this is how I would rank the most powerful Jedi and Sith. Abeloth is NOT included in this list because she is not a Sith, even though she uses the dark side.
Jedi:
1) Luke Skywalker - Unarguably the most powerful Jedi.
2) Jaina Solo - Defeated Darth Caedus, who was one of the only Sith to injure Luke at his full potential.
3) Yoda - Lived for 900 years, ranked first in knowledge of the force.
4) Mace Windu - One of the most skilled Jedi in lightsaber combat.
5) Obi-Wan Kenobi - Best practitioner of Soresu form.
6) Anakin Skywaler - Before the turn to the dark side. He was almost evenly matched with Obi-Wan, but lost the battle.
7) Kyp Durron - It is stated in one of the NJO novels (forgot which one) that Kyp Durron has more raw force power than Luke Skywalker, but lacks the control that Luke has.
8) Jedi Master Revan - Easily dispatched one of the members of the Dark Council in the novel, and lasted in battle for a reasonable amount of time against Darth Vitiate.
9) Jacen Solo - Killed Omini and ended the Yuuzhan Vong threat to the galaxy, and persuaded Zonama Sekot to join the fight. Could flow-walk and change the past.
10) Qui-Gon Jinn - Said to rival Mace Windu in lightsaber combat and discovered the child of the prophecy.
Sith:
1) Darth Sidious - Culmination of the Rule of Two.
2) Darth Plagueis - Darth Sidious' master. Possibly more powerful than Sidious, as the two never had a real final confrontation.
3) Darth Vitiate - Lived for over 1000 years, maintained the greatest Sith empire in history. Easily defeated Revan.
4) Darth Caedus - One of the most powerful Sith of all time. Defeated Kyle Katarn and 3 other Jedi Knights at the same time. Could use flow-walking to alter past events.
4) Galen Marek - Defeated Darth Vader in combat, and wielded force lightning that rivaled Sidious' in intensity. Ranked as a Sith because while his final goals were good, he still drew on dark side power (force lightning, force choke).
5) Darth Vader - Never achieved his full potential but still one of the more powerful Sith. Stated to have been 80% as powerful as Sidious.
6) Exar Kun - Took the entire Jedi Order to kill his body, and drove Luke into a coma for weeks while in a dark side spirit form that he had been in for thousands of years. No other Jedi or Sith has remained in the universe for that long after death to my knowledge.
7) Darth Tenebrous - Darth Plagueis' master, only defeated through trickery. Also possibly stronger than Plagueis or Sidious in force power.
8) Darth Bane - WITH orbalisk armor. Without it he is not nearly as powerful, though more cunning.
9) Naga Sadow - Could cause stars to go supernova with the force.
10) Darth Maul - Though not skilled with force powers, one of the best lightsaber duelists. Beat Qui-Gon Jinn, who was stated to rival Mace Windu.
Other notables:
- Raynar Thul
- Lomi Plo
- Kit Fisto
- Anakin Solo
- Kenth Hamner
- Saba Sebatyne
- Kyle Katarn