Oh man, this is easy.
Considering Voldemort's grand reign of terror ultimately amounted to running round Britain setting a few houses on fire and failing to take over a school, I'm going to go ahead and assume he gets taken out of the fight pretty quickly. For all he got called a 'terrifying Dark Lord' within the story, the actual evidence put him behind the IRA or the National Front in terms of actual death and destruction caused. And no prospective Dark Lord should have a murder count less than that of a group of pissed off Irishmen.
So, between Darth Vader and Sauron, I personally think it's an easy win: Sauron.
Sure, Vader has lightsabers and blasters, but Sauron has something that Vader couldn't hope to match. The ability to get inside people's heads and turn them insane, even from half the world away. Sauron consistently shows in the novels and films that he is able to reach out with his gaze and reduce the mind of anyone caught in it to a blubbering wreck. Even worse, Vader's already got a ton of mental baggage already stowed away in his mind. All Sauron would have to play Vader's own failings out in front of his mind's eye, alternating with blasts of 'Giant-Flaming-Eyeball', and the only thing Vader would be using his lightsaber for is hacking off his own head to put himself out of his misery. The brilliant thing is that we know Sauron can use this skill regardless of distance- he's able to fuck with Frodo and Pippin all the way from Mordor. Even if Vader was circling in orbit on his Death Star, Sauron would be able to fidn him and mind-rape him with his fiery eyeball.
Hell, that's only assuming that Sauron's in his 'floaty eyeball' form, not his 15 feet tall I'm-going-to-kick-the-shit-out-you form. If the latter, one pounding from that ludicrously huge mace is going to have Vader's lightsaber conking out from the stress, and Vader himself lying in pieces in a fifty food radius.
What about Vader's stormtroopers? Well, the Star Wars films themselves show that Stormtroopers are utterly incapable of hitting anything smaller than a barn door at anything greater than point-blank range, and even then their chances aren't great. A platoon of orcs with a few Trolls behind them would have no trouble wading through the woefully aimed laser fire, and utterly fucking up their shit.
Yeah, sorry. There's only one Dark Lord, and he's the one sitting at the top of the motherfucking Dark Tower, with the wraiths of undead kings as his generals.
/geekout