While the cover is beautifully illustrated by Carmine Infantino and Murphy Anderson, this, the first of Flash's 80 Page Giants within his own title (numbered Giant #G-21, or just Flash #160 of April, 1966, taking place after a Flash Annual and a couple of 80 Page Giants already devoted to the Flash), it really isn't a collection of his greatest super-speed rivals (at least not entirely)....
...but, take a look through the individual stories reprinted here and see of you agree.
Flash encounters a man faster than he is as Barry tries to stop a runaway truck. Problem is, the man doesn't recall who he is...so, the Flash tries to help him with his amnesia, as Iris West plans a charity race between the two (with the memory challenged fellow running backwards as he sees he is clearly faster than the Flash). Flash brings the man to a doctor, who after exposing him to electricity, does return his memory, but at the cost of his speed. The man is really an artificial lifeform called a Hominoid from another world, with the name of Kyri. Kyri was trying to return to his home dimension, but cannot without his speed. Flash helps him out, repairing his ship and providing the needed speed.
Isaac Bowen was a thief who got sent to prison in India, and partnered up with an Indian fakir, transferring the musical magic he learned to a fiddle instead of flute (using the fiddle to kill the fakir). Returning home, Bowen planned to use his new skills to become a crime boss, but instead ended up facing the Flash in Keystone City. It appeared at the end of this story that the Fiddler perished, but he would return (to face Flash individually as well as a member of the Injustice Society of the World).
But Fiddler really isn't a super-SPEED rival....
James Jesse premiered in this issue of the Flash, being a criminal who used gimmicks, including shoes that allowed him to walk on air, to commit robberies (all to make up for the fact that he was a failed circus acrobat...or at least had no affinity for that job; his fear of heights led him to invent the shoes that allowed him to walk on air, and his name, being the reverse of famous outlaw, Jesse James, led him to a life of crime). Trickster still had a bit to the showman in him, facing off against Flash, at least until his theatrics helped Flash figure out his identity, leading this act...to jail!
Don't worry, the Trickster does return, again and again, same Flash-time, same Flash-channel (but, also not really a speed threat....).
Wally's friend, Jimmy King, gets caught with an answer sheet to a test, and, leaves school. The teacher, Miss Grant, realizes her mistake and recruits Wally to find him. Jimmy went to New York to live with his cousin, Paul, the leader of a Beatnik Gang. Kid Flash busts the Beatniks during a robbery, then, Jimmy returns to Blue Valley and school, to clear his name.
Other than Kid Flash, little speed here.
Captain Boomerang gets out of jail, and has a new scheme with a special boomerang that travels through time, allowing him to establish an alibi for his crimes. Flash has problems solving this mystery, so calls in detective Elongated Man, and they sniff out the solution (but along the way, all three have to help stop an alien invasion, then have to contend with Captain Boomerang using a weapon gotten from those aliens against the Flash....with Elongated Man barely able to stretch to catch his friend in time!).
A great tale, but the rivals weren't all that speedy!
Adventure Comics #123
Johnny Chambers and Tubby Watts are headed to Africa to find the legendary Antelope Boy who can run so fast, but mobster Mobs Racket tries to get to the lad first. Shame he didn't count on having to outrun Johnny Quick and his magic formula. Johnny helps save the lad from the mobster, and is offered track scholarships to many schools in the United States!
Flash Annual #1 of 1963, it does lack one individual, the Reverse-Flash, who would most definitely need to be included in any collection of Flash super-speed rivals (but to be fair, at this point, Reverse-Flash only had 3 appearances!).