I would say Vader is the most powerful with conditions.
Against Sauron: Yes, mentally Sauron can do terrible things to a normal mind, however Vader does have the initial mental training and fortitude that he had from his Jedi training. Although Vader did fall from that path, his transformation into Darth Vader after Padmae's death and Obi Wan's betrayal killed any sense of hope or desire for peace. Plus, Sauron's mental corruption works with the ring of power, yes Vader desire's power but he also values knowledge about said power. If Vader simply does his research (like Gandalf did in LotR) then he can learn that the ring will eventually turn him into a slave. Granted, this makes killing Sauron difficult, however if Vader sends underlings to retrieve the ring (with oversight to prevent corruption), has them fly to Mordor (with friggin' ships with friggin' lazer beams!), Sauron can't really react. It does come down to armies here, and Orcs with machetes can't really compare with ATATs and the Nazgul with TIE Fighters.
Against Voldemort: Yes, Voldemort has several horcruxes to destroy in order to kill him, however an 18 year old kid with mediocre magical skills was able find and destroy the horocruxes. Face-to-face, Vader might have a disadvantage at a distance, but that disadvantage is matched by the huge advantage that Vader has at relative close range (50 meters). All Vader needs to do is get to that 50 meter range and it'll be the force and lightsaber against wand and sorcery. Vader's knowledge of the force allows him to avoid traps and fight against enemies at a distance. The armies are a bit of a wash and could go either way however because Voldermort has a physical body, he and Vader can square off.
Finally, look at who laid the final blow against these villains
Sauron, killed by Gollum and Frodo Baggins
Voldemort, killed by Harry Potter and Neville Longbottom
Vader, killed by Luke Skywalker and Emperor Palpatine.
In my opinion, it's a bit of a challenge but I'd give it to Vader.
Against Sauron: Yes, mentally Sauron can do terrible things to a normal mind, however Vader does have the initial mental training and fortitude that he had from his Jedi training. Although Vader did fall from that path, his transformation into Darth Vader after Padmae's death and Obi Wan's betrayal killed any sense of hope or desire for peace. Plus, Sauron's mental corruption works with the ring of power, yes Vader desire's power but he also values knowledge about said power. If Vader simply does his research (like Gandalf did in LotR) then he can learn that the ring will eventually turn him into a slave. Granted, this makes killing Sauron difficult, however if Vader sends underlings to retrieve the ring (with oversight to prevent corruption), has them fly to Mordor (with friggin' ships with friggin' lazer beams!), Sauron can't really react. It does come down to armies here, and Orcs with machetes can't really compare with ATATs and the Nazgul with TIE Fighters.
Against Voldemort: Yes, Voldemort has several horcruxes to destroy in order to kill him, however an 18 year old kid with mediocre magical skills was able find and destroy the horocruxes. Face-to-face, Vader might have a disadvantage at a distance, but that disadvantage is matched by the huge advantage that Vader has at relative close range (50 meters). All Vader needs to do is get to that 50 meter range and it'll be the force and lightsaber against wand and sorcery. Vader's knowledge of the force allows him to avoid traps and fight against enemies at a distance. The armies are a bit of a wash and could go either way however because Voldermort has a physical body, he and Vader can square off.
Finally, look at who laid the final blow against these villains
Sauron, killed by Gollum and Frodo Baggins
Voldemort, killed by Harry Potter and Neville Longbottom
Vader, killed by Luke Skywalker and Emperor Palpatine.
In my opinion, it's a bit of a challenge but I'd give it to Vader.