The rationale is a combination of overall popularity, originality, and fame as stretchers; but I went by feeling.
23. Parallel Elongated Men
The Elongated Man has several doubles from other dimensions of the DC Universe, so I decided to give them a spot. Among the most differently notorious, there are Elong-Gator, Elasti-Man, and his animated equivalent, the Extruded Man.
22. 'Tec #310's Rubber India Man
21. Elongated Kid
Honorable mention: Billy Warner
Although not officially, this character was the original "Elongated Kid". He didn't have stretching powers, but he is Ralph's #1 fan. He is also the closest thing to an Elongated Man recurring character (with 3 appearances).
19. The Elongated Evildoer
Actually, a hoax. But a classic Elongated Man story.
18. Malleable Man
A one-note villain who got Plas' powers and fought him along with Ralph, Elastic Lad and Superman.
17. The Stretch Armstrong supporting characters: Stretch X-Ray, Evil X-ray Wretch Armstrong, Stretch Monster, Stretch Ollie & Stretch Olivia, Fetch Armstrong, and Vac-man.
These characters kept elongating the Stretch Armstrong franchise throughout the years. The might get a bump from the upcoming movie.
16. Rubberband man
15. Stretch Dude
14. Elongated Woman
12. Monkey D. Luffy
Also interesting is that, like the Elongated Man, he also got his powers from a fruit, in this case one called "gumu gomu". As shockingly revealed in Bat-Manga: The Secret Story of Batman in Japan, after the sensation produced by the Batman 60s show, a Japanese series was created. What few people know is that the Elongated man also made it to that incarnation and a number of his Detective Comics features were adapted there, and get this: his Japanese name was "Gomu Ningen" (Gum Man). Gomu, as in Gomu Gomu, the fruit that causes the same effect as the Gingo elixir. How odd is that!
11. The Metal Men
Well, they are not rubbery at all, but they are definitively ductile and malleable. Their other abilities often distract from the fact that they are also stretchers. Like Ralph, these guys (robots, actually) are lighthearted DC silver age classics.
10. Professor Impossible
one other than Sir Dr. Stephen T. Colbert, D.F.A.
8. The Martian Manhunter
5. Mrs. Incredible / Elastigirl
4. Elastic Lad
3. Mr. Fantastic
2. Stretch Armstrong
1. Plastic Man