Enyalus
MALE DOMINANCE!!!
~~~~Vader's Assessment~~~~
Raw Power: This is fairly clear cut. Vader had the highest force potential in history, and would have possessed twice as much power as Palpatine shows at his peak. As it is, because of the injuries and his suit, it's canon that he possesses 80% of Palpatine's power. This places him at the high end of Sith Lords, although still below Caedus and presumably several others.
Force Mastery: This area is much more interesting. As far as learning goes, I don't know that there is much to say about him. We know he had a home at Bast Castle on the Dark Side steeped Vjun, that had at least one holocron Vader studied. He also had the most learned Force user ever as his master, and it's safe to assume that Palpatine taught Vader a great deal. It is clear from Palpatine's character, though, that he never taught him anything close to as much as the Emperor himself knew, nor anything that could aid Vader in seriously challenging Palpatine's rule.
Vader doesn't seem to have more exotic or "fun" Dark Side abilities like Force Drain or Force Lightning, which is sad for reasons I'll give in my conclusion. His mental force powers, though, are first and foremost among all Star Wars characters, in my opinion. Others may display a higher telekinetic feat (Nihilus, Sidious, Yoda, Kyp, Luke, et cetera), but no one shows such prowess and control of it time and again. Upon first learning of Padme's death, he unleashes a Force Scream that crushes/shatters everything in the medical bay. He becomes angry enough at Inquisitor Malorum that when Force Choking the Inquisitor, walls of the surrounding rooms were caving in. Mara once mused to Luke that Vader used the Force to throw an admiral through the wall of a ship because he showed a lack of respect for the Force. More impressive telekinetic feats would include effortlessly moving numerous metal objects – some of which were bolted down – without motioning his hand or arms to throw them at Luke, and Force Choking an admiral whom he had virtually zero connection to from miles away. Even better than that is the feat in the new Coruscant Nights book, in which he actually overloads Aurra Sing's lightsaber with pure willpower, rendering it useless.
His telepathy was sharp enough to read Luke's mind, as well as read the thoughts of all of his colleagues, the admirals. Moreover, he also proved capable of ripping thoughts from the minds of military-discipline trained soldiers (rebel pilots, at least). More impressive is his ability to sense his son's presence while aboard his ship, the Executor, even before Palpatine had. This is particularly enlightening when we consider that Palpatine already had knowledge that Luke would turn himself over to Vader, so presumably would have been attempting to monitor Luke's positioning. Another testament to his amazing mental force powers is his tremendous precognition, which helped him become one of the greatest pilots in the galaxy.
Lightsaber Ability: What can we say here. When Anakin first becomes Darth Vader, his lightsaber ability is great enough to rival anyone in the Jedi Order. He effortlessly destroys Darth Tyrannus prior to surrendering to the Dark Side (in doing so becoming even more powerful). His skill becomes great enough that he masters both variations of Form V, has a working knowledge of Form III and IV, and combines them all to form a customized version of Djem So. He was capable of easily defeating lightsaber combat droids – programmed in a dozen fighting styles and with the swordskill of a hundred masters, all with higher strength and speed of a normal human – even in two-on-one matches. His list of the Jedi he's defeated/killed/embarrassed is impressive, including Shadday Potkin, Koffi Arana, Ma'kis'shaalas, Roan Shryne, Empatojayos Brand, the Dark Woman (An'ya Kuro), Aurra Sing, Cin Drallig, Darth Maul, and Luke Skywalker to name a few. Which brings me to my summary:
Conclusion: The fact that Vader cannot versatile force attacks such as Force Lightning badly hurts him combat-wise. Force Choke and Force Grip are often easily negated by one's Force Shielding, and it is very rare to see one's defenses get shredded by purely TK methods. His telepathy isn't a worthy combat asset, either. Regarding his abilities with a lightsaber, which are profound – are several very low points:
He was almost killed at the Conclave on Kessel, in which was was ambushed by nine Jedi and needed help from the 501st Stormtrooper division to destroy them. This did not prevent him from being seriously injured in the encounter. While on the planet of Shumari, he is viciously attacked and severely wounded by a Tusken Raider, Vader's failed apprentice on the verge of death having to save his life...He is able to overcome Boba Fett, although with great difficulty – Boba having several opportunities to win during the battle. On Mimban, Vader dueled a literal neophyte Luke with zero lightsaber training and got his arm hacked off. (Granted, Obi-Wan's spirit was guiding.) Vader was again almost killed by the bounty hunter Awarru Tark, barely surviving the encounter and coming away with yet more scars to his already deformed face. When Darth Maul is resurrected by the Prophets of the Dark Side, Vader seems to be evenly matched. However, once Maul is forced to revert to Jar'Kai (a style with which is not his preference), Vader becomes quickly overwhelmed and is once again about to be killed if not for plunging his own saber through his body to impale Maul – severely damaging his life support systems in the process. Added to this is the fact that during their duel on Bespin, Luke (again, with virtually zero lightsaber combat training) is able to get the better of Vader twice – once kicking him off of the carbonite platform and once striking him on the shoulder with his saber. Besides that, we all remember him being overpowered by Luke, tapped into the Dark Side, on the Death Star. Not to mention his loss to Galen Marek, in which the novelization portrays Galen as decidedly beating Vader handily.
Once everything is taken into consideration with regards to Darth Vader, I believe the final conclusion is that while he had a tremendous amount of power (Sidious believed that it wasn't the suit which held his potential back, but his own psychological weaknesses), he is unable to utilize it to the best of his abilities, being badly encumbered by such a weighty suit and having badly glaring weaknesses as the life support system. He has the raw power to stand toe-to-toe with some of the most powerful characters in the mythos, but due to the limitations of the suit, I do not think he has the lightsaber ability to contend with many of them.
I will finish my thought regarding DE Sidious' power tomorrow evening - I think you all will be entertained.
EDIT: I haven't proof read this yet. It's almost 1 AM here and I got up at 6 this morning, and my brain is just slightly not working correctly, so I hope everything is clear and not backwards *is dyslexic*