The Arrowverse is absolutely packed with super-speedsters. Barry Allen may consider himself the fastest man alive, but he's far from the only super-speedster in the Multiverse. Ever since that fateful day he was struck by lightning, the Flash has found himself confronted with countless friends and foes who seem to match his speed.
From an out-of-universe perspective, superhero comics and TV shows have long enjoyed pitching heroes against twisted mirror-image enemies. In the case of the Flash, the writers seem to have initially struggled to imagine scenarios where Barry Allen could really be fought by anyone who didn't have the same kind of power set; after all, he can potentially take out any threat so quickly they don't even see him coming. The show became more creative as it went on, realizing it had fallen into a repetitive trope.
Of course, The Flash has indulged in both time travel and the concept of the Multiverse, meaning speedsters come from countless different times and dimensions. The following list is all the characters in the Arrowverse with speedster abilities; it doesn't include characters like Trajectory, Savitar, or Zoom, who have been killed, or speedsters mentioned in tie-ins that may not be canon.
The Flash - Barry Allen (Earth-1)
Barry Allen himself is the first, the best, and undoubtedly the fastest. He has an intimate connection to the Speed Force, created when he was struck by dark matter lightning. At one point, the Flash spent six months trapped within the Speed Force, and his cells were saturated with the Speed Force, permanently altered and boosting his powers; later, Oliver Queen — as the Spectre — apparently enhanced the Flash's speed even further. Barry's previous top speed has been measured at 1.25 percent the speed of light, but he's now probably able to exceed this by far.
The Flash - Barry Allen (DCEU)
Crisis on Infinite Earths featured an unexpected cameo from Ezra Miller's DCEU Flash, confirming the DCEU is part of the same Multiverse. It was the ultimate piece of fan-service, bringing Miller and Grant Gustin face-to-face, and even suggesting Earth-1's Barry Allen gave his DCEU incarnation the idea for his codename. It's unclear quite how this works, though, because technically the encounter took place after the destruction of the Multiverse, meaning it really should have been impossible.
Kid Flash
Next up is Wally West, a.k.a. Kid Flash, the son of Joe West and Iris' younger brother. He was exposed to the Speed Force in the Flashpoint timeline; although the timeline was reset, Wally regained his connection to the Speed Force when he encountered the Philosopher's Stone. Wally is one of the most powerful speedsters around, with his powers growing at a far greater rate than the Flash's, although it's not clear whether that's still the case. Some of Wally's powers do seem to vary a little when compared to other speedsters; his super-healing is particularly inconsistent.
Reverse-Flash
Eobard Thawne is the most dangerous super-speedster of all, born in the 22nd century and obsessed with the Flash. He successfully recreated the accident that gave Barry Allen his powers, and became a speedster himself. Thawne became the Reverse-Flash, and he learned to tap into a new power called the Negative Speed Force. This is the yin to the Speed Force's yang, an extradimensional field of negative energy that eats away at the Speed Force like a cancer. It's clear a speedster who accesses the Negative Speed Force can use it to do everything a normal speedster does with the Speed Force. Following the reveal that Thawne merged into all versions of Wells post-Crisis, his current status in the Arrowverse is unknown.
Nora West-Allen
Nora West-Allen originates from a future timeline in which the Flash died during Crisis on Infinite Earths, and she traveled back in time in order to meet her father. Unfortunately, Nora unwittingly rewrote the timeline, ultimately leading to her being erased from reality itself. It's possible the aftermath of Crisis on Infinite Earths has restored Nora to existence, but in this new timeline she would hopefully have grown up with Barry Allen to help bring her up.
Jay Garrick
The Flash of Earth-3, Jay Garrick styled himself as the Crimson Comet, and he's been a frequent ally of Team Flash. He understands the Speed Force better than probably any other speedster bar Reverse-Flash, and during the buildup to Crisis on Infinite Earths, he was the only one who successfully detected the antimatter waves destroying different realities. Earth-3 was obliterated, but Shipp has indicated Jay could return. Curiously, Jay did mention training a protege, an unknown super-speedster.
Jesse Quick
Jesse Quick is the daughter of Harrison Wells from Earth-2, and her Speed Force connection was unlocked by an antimatter wave from the STAR Labs particle accelerator. She's been a frequent ally of Team Flash, even moving to Earth-1 for a while and dating Wally West. Unfortunately, after Savitar was defeated she traveled back to Earth-2, and was living there when her dimension was struck by an antimatter wave. It's unknown whether she was restored when Oliver Queen restored the Multiverse.
The Accelerated Man (Earth-19)
It's easy to forget the Accelerated Man, who lives on Earth-19. Assuming he's the same as his comic book equivalent, he's an alter-dimensional Barry Allen; the Accelerated Man has only been encountered a couple of times by dimension-hoppers such as Gypsy, and it's impossible to say whether he still exists in the post-Crisis universe.
Blitzkrieg (Earth-X)
Another exotic one, Blitzkrieg has only appeared in the animated miniseries Freedom Fighters: The Ray. He's the speedster of Earth-X, a ruthless member of the New Reichsmen, and he's enhanced his abilities as a speedster by using a rapier as a deadly weapon. He was captured by the Freedom Fighters, and hasn't been seen since.
Godspeed
Godspeed is probably the most confusing of all the super-speedsters to date. He originates from Nora West-Allen's future timeline, where Nora beat him after being trained by Eobard Thawne; after Nora was erased from the timeline, though, he began sending speedsters back in time to cause problems for Team Flash. This appears to have stopped after the events of Crisis on Infinite Earths, presumably meaning Godspeed's timeline has changed again.