Voldemort, probably. Vader will find it very difficult to defend aginst Voldemort (killing curse among other things), and if Voldemort makes good use of his teleportation, Vader probably won't be able to do jack to him. However the force could possibly give Vader the advantage. It's very hard to determine.
Doesn't Voldemort need spells? In that case Vader could just choke him or break his neck, before he can spell a word... He will also be able to sense where he teleports...
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Before this gets closed I'll say Voldemort for either Suit or Human Vader. But if Vader could just get that saber toss or Force Push or Choke...
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You don't need to speak the words without casting a spell. I don't know about the Avada Kedavra specifically, but a great deal of other harmful spells are in Voldemort's array.
__________________ Recently Produced and Distributed Young but High-Ranking Political Figure of Royal Ancestry within the Modern American Town Affectionately Referred To as Bel-Air.
Vader quickly raises his left hand and force chokes Voldemort while executing a lightning fast flick with his right to send his Lightsaber flipping through the air. The crimson light of the blade whirls through the night, spilling its bloodshine across the ground as it speeds towards it's target. Recognising the danger, Voldemort hastily extends his wand and with the last ounce of his strength, forces wind through his rapidly decreasing larynx.
"Avara Kedavra"
The blast hits Vader directly in the torso and the terrifying sound of his electrically assisted breathing is suddenly cut off. The silence following this shattering developement is almost tangible and seems to weigh heavily on those present as they watch the Dark Lord of the Sith slowly fall forward onto his knees. Yet, even kneeling in defeat, his ravaged body dead in the heartbeat that the curse slammed into him, the twin reflective black pools, set as eyes within the helmet that struck fear into an infinite number of souls on countless worlds, display one final moment forever imprinted on those who witnessed it.
As gravity finally claims the ruined husk of the man that had once been a prodigious child pilot, a frustrated and misunderstood padawan, a roguish and cunning Jedi Knight and an altogether vicious and merciless Lord of the Sith. It is not the sight of the final fall this man is making, it is not the way that time seems to have slowed as the once overwhelming body now topples forward onto the ground, to lie still forever. No, the image that will forever be burned into the minds of all present is this.
For the vision displayed for only that short period of time, is the reflection of Lord Voldemort, his proud, haughtly yet slightly reptilian face is a study of shock. For lying, now discarded in the grass behind the wizard, is the hilt of a deactivated lightsaber.
This is actually a really close battle.
Both combatants have extremely powerful attacks imo; Vader - choke, Voldemort - killing spell. Vader's connection to the force could actually grant him victory, but I'm still uncertain whether Vader would be able to handle Voldemort's teleportation. Voldemort also likely has many more abilties that we don't know of, and he can only become more powerful in the next book while Vader's power is basically definite.
With an almost inaudible sigh, the being that was once Tom Marvolo Riddle, heir of Salazar Slytherin, and commonly known to all as Lord Voldemort, slowly sags and then splits along a seam that stretches from his left shoulder to his right hip. These two halves of what was once whole seem to almost crumple like a house of cards that has lost its support.
For even before the Dark Lord of the Sith had begun his final fall from grace towards an unforgiving and uncaring ground. Those twin portals of darkness had already bore mute testimony to the death of his final enemy.
Without a single word being spoken, those that were present, and bore witness to such a battle of titanic proportions, turned away as one. Almost reverently they departed the site of this final battle of Dark Lords. Neither caring to paw over the remains of the dead or to view any further the two motionless forms, of what were once the most feared beings in the galaxy. Instead trusting to nature, to the wild, to slowly reclaim and forever hide within its lush foliage all evidence of the events of this night.