I don't claim to be an expert on the NFL draft or evaluating prospects, but I have been following the draft process for decades now, and feel like I can weigh in on this question in proper context.
That is -- without relying too much on hindsight. For example, J.J. Watt (the #11 pick in 2011) went on to become a Hall of Fame caliber player, but back in 2011 there were still some lingering doubts about him. J.J. Watt the player may be one of the best pass rushers of all-time, but J.J. Watt the prospect was not.
This list serves to rank the edge rushers AT THE TIME they were prospects. Another way to consider this is: if all the players were in the same draft pool, who would be selected first? It's a difficult endeavor, but here's my best estimation.
#players who missed the top 10
DEMARCUS WARE (#11 pick in 2005): Ware had some rabid fans, but he wasn't a top 5 pick and super prospect largely because of his small school status.
KHALIL MACK (#5 in 2015): Similarly, Mack fell a little too far because he played at small school Buffalo. Savvy evaluators were on to him, and knew his staggering 74.5 tackles-for-loss over 4 years was more indicative of his future talent than his more modest 28.5 sacks, but that wasn't a common insight at the time.
VIC BEASLEY (#8 in 2015): I ranked Beasley as my #2 prospect in 2015, because he was a highly productive player who confirmed his athletic prowess at the combine. However, some detractors (notably Todd McShay at ESPN) questioned his functional power.
DANTE FOWLER (#3 in 2015), GAINES ADAMS (#4 in 2007): Both of these rushers were top 5 picks, but were seen as a little bit of a "reach" at the time. In a more stacked class, they would have slipped further.
JUSTIN SMITH (#4 in 2001), TYSON JACKSON (#3 in 2009): Another two high-end picks that wouldn't crack my top 10, because they projected as more two-ways players than dominant pass rushers, even at the time they went in the top 5.
#The Top 10 Edge Prospects (at the time)
(10) Vernon Gholston, Ohio State, #6 in 2008
This is a good time to remind you, again, that this is based on their draft stock at the time. In hindsight, we remember Gholston was a workout wonder, but that's only half true. There's no doubt that Gholston became famous based on his shredded physique and freaky measurables -- he ran a 4.58 at 266 pounds, jumped a 41" vertical, and threw up 37 reps on the bench. However, he was also a productive player for 2 of his 4 years at Ohio State, registering 8.5 sacks as a junior and then 14.0 as a senior. Even now, I wonder if Gholston may have been better off on a different team that planted him as a 4-3 end, rather than trying to convert him to a 3-4 rush linebacker as Rex Ryan and the Jets did.
(9) Joey Bosa, Ohio State, #3 in 2016
Players like Gholston (who take 2 years to finally rev up their production) should be a red flag. That certainly wasn't the case with Joey Bosa, who was productive immediately at OSU, logging 13.5 TFL and 7.5 sacks. Bosa went on to finish his 3-year career with 51.0 TFL and 26.0 sacks, which should have locked him into elite status. However, there were a lot of questions about Bosa's athleticism (after he tested at 4.84 in the 40 at 269 pounds). He was seen as an A+ player but a B+ athlete, and consequently would fall behind the higher ranked players here.
(8) Dion Jordan, Oregon, #3 in 2013
Athleticism was not Dion Jordan's problem. Like Vernon Gholston, Jordan looked absolutely shredded, and exploded with a 4.54 40 at 6'6" and 248 pounds. At the time, the NFL was falling in love with speed, and the converted WR Jordan fit the bill. In fact, the Miami Dolphins mortgaged picks to trade up to #3, with the expectation that they'd address their need at OL and draft Lane Johnson. Instead, they went with Jordan, indicating how valued he was at the time. In hindsight, the fact that he was only productive in 2 of his 4 years at Oregon should have been a concern. That, and the fact that his weight/muscle gain may have been aided by the darker aisles of GNC.
(7) Chris Long, Virginia, #2 in 2008
Unlike Jordan, Chris Long wasn't a quick, twitchy athlete, measuring out in between Jordan and Bosa, with a 4.75 40 at 6'3" 272. However, scouts absolutely loved Long's game at the time. Despite playing in a 3-4 that should have limited his stats, Long demonstrated a great ability to play the run (79 tackles as a senior) as well as "plus" pass rushing ability (14 sacks in 2007). The biggest feather in his cap was his pedigree. Like Eli Manning and Jake Matthews, Long got a boost by his family bloodlines. The fact that he nailed a 34 in the Wonderlic further suggested that he was a smart, well-rounded player who wouldn't fail.
(6) Von Miller, Texas A&M, #2 in 2011
Based on his pure tape, Von Miller was the greatest natural pass rusher of anyone in the bunch. Miller could bend around the edge better than anyone. A few scouts described him as almost superhuman in that regard. Still, it'd be hindsight to put Miller in our top 3 here. He was fast (4.53 40) but lighter in the pants than other super prospects at 6'2" 246. That, and the fact that he had only excelled in 2 of his 4 years at Texas A&M make him slip a few spots.
(5) Jadeveon Clowney, South Carolina, #1 in 2014
Clowney's athleticism and explosive ability made him a legend from day one in college. He confirmed that at the combine, lighting it up with a 4.53 40 despite being 6'5" and 266 pounds. However, there were questions about Clowney, even if he turned out to be the #1 pick. People questioned his effort and his junior year production (3 sacks in 11 games).
(4) Myles Garrett, Texas A&M, pick #?
Here's where I'd rank Garrett among this stellar group. In a way, Garrett combines so many great traits of these top edge rushers before him. He has great size (6'5" 270), and blew up the combine (4.64 40, 41" vertical). Garrett also backs it up with production. Like Joey Bosa, he immediately produced as a true freshman, logging 12.5 TFL and 11.0 sacks, and went on to log 47.0 TFL and 31.0 sacks in his three years. He may not have the explosive ability of Clowney and Von Miller, but his extra bulk, size, and production makes him the superior college prospect.
(3) Mario Williams, North Carolina State, #1 in 2006
Reggie Bush was expected to be the #1 pick in 2006 (back when RBs could actually do that sort of thing), but the Texans couldn't deny the freaky talents of the mammoth Super Mario Williams. NFL fans in Buffalo and Miami may grumble about Williams now, but at the time, the guy was an absolute beast. He ran a 4.73 40 and pulled off a 40" vertical -- despite measuring 6'7" and 295 pounds. Let me repeat: 295 pounds. Williams carried that weight well, and still managed to be a heck of a pass rusher (with 24 TFL and 14.5 sacks as a junior). The fact that the Texans were criticized for taking an edge rusher over a RB at the time should tell you a lot about how far the NFL has evolved.
(2) Julius Peppers, North Carolina, #2 in 2002
Big Julius Peppers was a similar prospect to Mario Williams. He combined enormous size (6'6" 283) with great athleticism (4.68 in the 40). Like Williams, Peppers was also a productive pass rusher, amassing 15 sacks as a sophomore. What separated Peppers from Mario Williams and Myles Garrett was the perception that he still had untapped potential. Peppers was a multi-sport athlete at UNC, playing with their stacked basketball team. Given that, the thinking was: he'd get better and better in the NFL, soaring to unprecedented heights.
(1) Courtney Brown, Penn State, #1 in 2000
Wait, who? There was a prospect better and freakier than Jadeveon Clowney? Better than Julius Peppers? And he was a BUST? How does this happen?!
Settle down kids, and let me tell you a tale that you won't believe. At the time, Penn State had a stacked defense, led by LaVar Arrington (who may have cracked this top 5 if I considered him a pure edge player). But before long, their DE Courtney Brown emerged as the better, cleaner prospect of the two.
It's easy to see why. Courtney Brown simply had no flaws. He was a chiseled 6'5" 270 pounds, but he blazed a 4.52 in the 40 yard dash. A 4.52. At 270 pounds. He was stellar in the run game, but also a great pass rusher, logging a team record 33 sacks and 70 TFL in his 3 years on campus. As if Courtney Brown couldn't be more of a "can't miss" prospect, he was also supposed to be a great kid. He famously carried a 4.0 GPA throughout school. In fact, people weren't talking about Brown as an inevitable Pro Bowler, but as an inevitable Hall of Famer.
Obviously, that never came into fruition. Brown struggled with injuries (and some believe a lack of tenacity and aggression) that made him more a solid run stopper than a demon on the field.
But this post isn't about the best pass rushers ever; it's about the best prospects. Brown, at the time, would have been my #1.
conclusion
Obviously, people can quibble with this list and these rankings, so I'd be curious about where you'd place these rushers. Whether you put Garrett a few spots higher or lower, I think we can all safely say that he's one of the top 10 of the 2000s, which should make Browns fans very happy. (Or 49ers fans, should the Browns get Brown-y.)