Darth Vader, a former heroic Jedi Knight, was seduced by the dark side, which resulted in him becoming an all powerful Sith Lord. Vader led the Empire's eradication of the Jedi Order, while he remained in service of the wicked Darth Sidious. The Sith Lord is feared by an entire galaxy, and has been a thorn in the backside of the Rebel Alliance for years. Darth Vader seems to lack mercy, as he force chokes the life out of his own men, and anyone who dares to stand in his way. But is he unstoppable? Not against these guys..
Wolverine is one of the baddest, meanest characters of the Marvel Universe. He's survived his flesh being torn from his skeleton, and has kept himself alive while underneath a glacier, by eating his own flesh. Now that's tough. The Wolverine's primitive instincts and berserker rage add volume to his already spectacular abilities. He's got a near indestructible skeleton, a healing factor, and mad experience in many forms of martial arts. The mad mutant could surely take down Vader with the help of a team.
Emperor Zurg (Life-Sized)
Evil Emperor Zurg is basically Darth Vader's equal. Well, sort of. He is an enemy of the Galactic Alliance, and the sworn enemy of Buzz Lightyear. Zurg is a master planner, as well as an evil genius. The space emperor's resources and his thirst to control the entire galaxy could surely come in handy in the fight against Vader.
Other than Wolverine, Magneto is probably one of the angriest mutants of them all. He's got a life full of resentment, betrayal, and pain to show for it. He can manipulate metal, generate magnetic force fields, and is granted flight through magnetic fields. His helmet keeps him safe from telepathic attacks (*ahem* Jedi mind tricks) and can fight Vader's attempts to take his helmet off, by using metal manipulation. Magneto is like the Hulk in a very small, yet noticeable way. The angrier he gets, the stronger he gets. This extremely powerful mutant could definitely lead the attack on the Death Star, succeeding in taking down the Sith Lord.
This guy is one heck of a character. You thought Vader lacked mercy? Meet Lobo. The bounty hunter from space is the last of his kind by his own fault; he committed complete genocide by killing all of his people, just for fun. His name translates into "He who devours your entrails and thoroughly enjoys it." Lobo isn't only Stan Lee's favorite DC character, but he's a super-powered genius (surprised?) who would be a great help in taking down Lord Vader.