In the magical world of Harry Potter, there are few organizations or groups that strike fear into people's hearts more deeply than the Death Eaters. The intimidatingly named Death Eaters were the dark witches and wizards who followed Lord Voldemort and hoped to turn the wizarding world into an anti-muggle, morally laissez-faire nightmare for most normal people and magical folk. And while the Death Eaters were almost as frightening as Lord Voldemort himself, obviously some of them were more intimidating and dangerous than others.
Sam and Dean Winchester clearly don't exist in the world of Harry Potter, because if they did then Voldemort probably would have taken a bullet to the head courtesy of John Winchester decades ago. However, if they did cross fictional paths, which Death Eaters could Sam and Dean defeat without a problem, and which Death Eaters could actually pose a threat to the Winchester Boys?
Could: Igor Karkaroff
Igor Karkaroff may have been one of Lord Voldemort's devoted Death Eaters when the Dark Lord was actually around, but once the head of the snake had been metaphorically cut off, Karkaroff was all too eager to flip on his fellow criminals in order to avoid imprisonment himself.
So Sam and Dean Winchester would have less than no problem defeating Karkaroff. Igor is the kind of man who thinks he has a lot of big ideas and thinks he's quite tough, but when the rubber hits the road he will do whatever it takes to protect himself.
Couldn't: Regulus Black
Well, on the plus side, Regulus Black may be one of the very few Death Eaters Sam and Dean wouldn't actually want to defeat, even if none of them knew it at the start of things. However, it seems like the Winchesters would have had a hard time spotting the fact that Regulus was such a real threat.
The Dark Lord himself certainly didn't, and he was undoubtedly paranoid and suspicious of his followers from time to time. Voldemort and the Winchesters clearly have very different skill sets, but anyone who is clever enough to trick the former will likely be able to trick the latter.
Could: Fenrir Greyback
Technically Fenrir Greyback was never made an official Death Eater, but he was a more active participant in Lord Voldemort's activities than many actual Death Eaters, and most Harry Potter fans aren't even aware that this beastly man never got his Dark Mark.
And although Greyback was likely one of the most dangerous and difficult to face adversaries for the average witch or wizard, we think that Dean and Sam would have been able to handily deal with Fenrir. Unlike most Death Eaters, Greyback was more of a hand to hand combatant, which is probably a much easier fight for the Winchesters to win.
Couldn't: Bellatrix Lestrange
Bellatrix Lestrange is undoubtedly Lord Voldemort's wildest and craziest follower. Sam and Dean Winchester wouldn't be able to follow her frenetic behaviors to begin with, and her willingness to murder anyone who inconveniences her would put them in a significantly more dangerous place than they'd be in with most other Death Eaters.
Usually people can reason with evil, but there is no reasoning with insane, and Bellatrix has been beyond reason for decades now. She's beaten some pretty tough witches and wizards in her day, so she'd probably be able to take down the Supernatural bros too.
Could: Lucius Malfoy
It would honestly be embarrassing if the Winchesters couldn't beat someone like Lucius Malfoy. Yes, he is technically a Death Eater, but in reality he is nothing more than a spoiled brat who uses any situation to his advantage, even if it's evil.
His commitment to the cause is only as deep as the potential benefits that he can reap, and clearly when things get too real he completely falls apart. By the time the war was near it's end, Lucius was over being a Death Eater anyway, so with a little effort Sam and Dean could have undoubtedly crushed him like a bug.
Couldn't: Narcissa Malfoy
If anyone in the Malfoy family actually deserves to have some respect put on their names, it's Narcissa. She's not any more benevolent than the rest of the bunch, however she is tougher than Lucius and Draco combined.
What makes Narcissa such a brilliant and dangerous adversary is that no one would expect her to be strong, bold, or tough, but she is actually all of those things. Lord Voldemort isn't always the best at reading people (even though he thinks he is), but most people are too terrified to even bluff him. Narcissa is not, and she would give Sam and Dean a run for their money as well.
Could: Draco Malfoy
Is it humanly possible for anyone to be more unlike the Winchester boys than Draco Malfoy himself? Spoiled, weak, and only out for himself, Draco Malfoy always thought that being a Death Eater would be super cool, and he even used his Dark Mark to flex on the other kids at Hogwarts.
However once he was tasked with the mission of killing Albus Dumbledore, things got a little too real and Draco completely fell apart. He is undoubtedly one of the easiest Death Eaters to face off against, and he is the inferior of Sam and Dean Winchester in every imaginable way.
Couldn't: Barty Crouch Jr.
Barty Crouch Jr. may be a few sandwiches short of a picnic, but credit where credit is due, he can really pull a fast one on people when he puts his mind to it.
Barty doesn't seem like the kind of person who could pull off a long and detailed plan that requires an enormous amount of patience and forethought, but he did well enough at portraying Mad-Eye Moody for an entire school year that no one ever once suspected that he wasn't the legendary auror until it was too late. If he could defeat someone like Mad-Eye, he'd have a good shot against the Winchesters too.
Could: Peter Pettigrew
If there was ever a Death Eater that Sam and/or Dean Winchester could defeat, blindfolded, with both hands tied behind their backs, then that would be Peter Pettigrew. Peter Pettigrew can barely defend himself from a squad of children (they may be the golden trio, but they're still basically 3rd graders) even on his best day, and on his worst day he's a literal rat.
If Sam and Dean found Peter in his rat form then obviously they could kill him with one stomp of a foot, and if they ever had to face off against human Peter he'd probably just cower and beg for mercy.
Couldn't: Severus Snape
In the end, the Winchesters would have undoubtedly been relieved that Severus Snape was actually on their side all along. And the fact that Snape managed to play two of the strongest wizards who ever lived at the same time and make both of them fully trust in his loyalty would strongly indicate that he could earn the trust of the Winchesters too.
Sam and Dean are hunters, not necessarily strategists, and Severus really knows how to play the long game better than anyone. The Winchesters wouldn't be able to beat Severus, but they wouldn't want to either.