Sauron 1v1 -
Strengths - He is a lesser god-being. Thousands of Years of Experience, knowledge
Weaknesses - He put his main power into a ring. He is pretty much running around gimped from that.
Voldemort 1v1 -
Strengths - He is a strong mortal sorceror, but has 7 things you have to break before killing him.
Weaknesses - I don't think he can resist Sauron's mental corruptions. Saruman was thousands of years old master wizard who couldn't.
Vader 1v1 -
Strengths - Better at physical combat.
Weaknesses - Force abilities lack the unlimited range and power of magic. He could probably resist Sauron's mental attacks, and kill Sauron.. I don't know if he could do anything about Voldemort who is not outright slayable to any method Vader has.
Sauron's Army -
Numbers : B - Orcs seem to outnumber anyone else on Middle Earth.
Skill : D - Orcs are not really skilled, almost no orc is a match for a human soldier in a single combat.
Equipment : F - Orcs have shoddy medieval weapons.
Vader's Army -
Numbers : A - Recruiting an army from vat grown clones or recruiting them from a galaxy-spanning empire. Either way, Vader has this on lockdown.
Skill : F - "Only Imperial Stormtroopers are so precise".
Equipment : A - Well, Star Wars level of Technology assures them victory.
Voldemort's Army -
Numbers : F - A sect of rogue mages primarily in England, their numbers can't be more than a few thousand.
Skill : A - They actually know what they are about. Individually, they can threaten main characters.
Equipment : C - Modern and Magical equipment is fairly potent.
Army Battle Conclusions -
Vader will win any war, Voldemort's Army would win any light skirmishing or raiding. Sauron's army would roll over and die.
1v1 Conclusions -
Vader will win against Sauron because Sauron doesn't do overt magic really and Vader has a weapon that can chop off the ring.
Vader will lose against Voldemort because Voldemort generally can't be killed.
Sauron will win against Voldemort because Sauron can't be killed by Voldemort's magic, and can destroy Voldemort's mind with his more dominating mental presence.