The Flash is the Fastest Man Alive, as we know. When it comes time for DC to create villains for the hero, then, it's easy to see the idea of fellow speedsters rising to the forefront. In fact, The Speed Force became so prominent that the Flash TV show had to make a change after a few seasons, to keep speedsters from dominating the series.
However, in the comics, there were plenty of chances for other antagonists --over almost 80 years of publication and various villains-- to take on the Scarlet Speedster. In that world, the Speed Force was still important; there were many villains who came along to challenge Flash to the title of Fastest Man Alive (without overpowering the story). Here's a selection of them.
Inertia is Thaddeus Thawne, the speedster clone of Bart Allen and President Thawne from the 30th Century. His development in the use of speed was very slow, so Inertia became more calculating and worked hard to become even smarter than Bart Allen at the time (a hero who was able to develop very fast in comparison).
He was a more recent addition to the rogues' gallery, a nemesis for Bart, who made his debut in 1999. His super speed is achieved through a process called localized chronokinesis, which allows him to alter time to run at speeds rivalling those of speedsters who use the Speed Force (or appear to).
Rival is Edward Clariss, an enemy of Jay Garrick, the first Flash. He gained his speed by attempting to recreate the formula that delivered Garrick's speed powers to him. He started to become a criminal when people wouldn't accept the claims of his scientific discovery.
However, while Rival's formula worked and gave him super speed that rivalled that of Flash, it was only temporary. When the formula wore off, his speed disappeared. However, he tried again and actually hit light speed this time around, causing him to disappear into the Speed Force. He returned years later as pure speed energy, since he spent so much time in the Speed Force.
8 Future Flash
Future Flash is a version of Barry Allen from an alternate future, who was ravaged with guilt when Wally West died and he couldn't save him. Due to this tragedy, Barry chose to try to fix all the problems of his past by racing through time --serving as either an executioner or a judge to his rogues' gallery-- while also trying to fix a ruptured Speed Force by any means possible.
Because this is Barry Allen from a possible future, he has the same speed as Barry. This version also wears a cybernetic suit that enhances his powers. While this version of Barry is the villain of the story --especially with the casualties and the attempt at taking his own life in the past-- he ends up being saved by Wally.
7 Dark Flash
Walter West is an alternate world version of Wally West, but with one major twist. While Wally West was able to save Linda Park's life, Walter was not. As a result, he developed a very dark personality. Instead of being a legitimate superhero who saved people, Walter became Dark Flash and brutally assaulted criminals, crippling many of them.
As an alternate version of Wally West, the Dark Flash shares the same speed as his counterpart and is fast enough to be in three places at the same time. Walter was more powerful than Wally when they met, as he had more time to study the Speed Force.
Savitar was the main villain in the third season of CW's The Flash. In the comic books, Savitar is a very powerful cult leader that rules over people who worship the Speed Force. He has fought just about every version of Flash and is named after the Hindu god of motion.
Things changed on the TV show. In the world of the Arrowverse, Savitar was Barry Allen from an alternate future, in which he had no loved ones and became a villain. As a result, he had the same speed and powers as Barry, in much the same way that Dark Flash was an evil version of Wally West.
5 Black Racer
Black Racer is not just one of the fastest characters in DC, but he is one of the manifestations of Death itself. He was a servant of Darkseid, one of his greatest weapons, until he was finally freed and combined with Barry Allen. When that happened, he turned on his long-time master.
Black Racer and Flash were finally separated, and it was discovered that the being could merge with any other superpowered lifeform. While that was the origin of the New 52 version, there was also the original Black Racer, which was part of Jack Kirby's New Gods. It was the fastest bring in all existence, able to even outrun Death.
4 Hunter Zolomon
Hunter Zolomon is better known to the general public as Zoom. In the comics, he is the third man to take the role of Reverse Flash, a former friend of Wally West who became consumed with rage after Gorilla Grodd crippled him. He gained the powers of a speedster after an accident with the Cosmic Treadmill, becoming one of Wally's greatest enemies.
There was also a version of Hunter in the Arrowverse, one of the antagonists for season 2 of Flash. He was from Earth-2 and wanted to be the only speedster in the entire multiverse. When it comes to speed, Hunter could go toe-to-toe with Wally West. At one time in the comics, he was fast enough to stop the Justice League and Titans before any of them could react to his attack.
In the comics, Godspeed is August Heart, Barry Allen's partner. He was a witness to the accident that gave Flash his powers. August was later struck by lightning, going on to eventually become Godspeed, a ruthless vigilante who wanted to achieve vengeance for the death of his brother.
August actually gained so much of the Speed Force that he was faster than even Barry Allen, although he lost much of that power at the hands of Flash and Kid Flash. At one point in his career, Godspeed was so fast that he could actually decapitate someone with just a single punch to the head.
2 Eobard Thawne
Eobard Thawne has been known as both Reverse Flash and Professor Zoom, easily the fastest member of Flash's rogues' gallery and his most dangerous villain. He is from the 25th Century and has an intense hatred for Flash, making it his life goal to destroy Barry Allen. Thawne was the man who murdered Barry's mother and framed his father, which also made him responsible for Flashpoint.
Barry Allen has said that Thawne is faster than both he and Wally West, which makes Thawne one of the fastest villains that the Scarlet Speedster has ever tangled with. Thawne was also the villain in the first season of Flash and was much faster than Flash on the television show as well.
1 Black Flash
While Eobard Thawne is the fastest villain Flash has ever faced, Black Flash is the fastest speedster of all-time. This being is Death for anyone who possesses the power of the Speed Force. When he shows up, someone is about to die, as we've seen in the cases of Barry Allen, Johnny Quick and Bart Allen.
This being comes for those whose time is up and is faster than them all. The creature can run faster than the speed of light and easily kept up with Wally West with little effort. At one time, both Barry Allen and Eobard Thawne took on the role of the Black Flash and gained increases in their speed as a result. Hunter Zolomon took on the role on the TV show.