Source: Various (see below)
Type: AnalogsThough Mon-El is the longest lived of the superhuman teens who visited Smallville during Superboy's career, there are many others who tried to steal his thunder, his place or his girl. It's one of the tropes of the 50s and 60s, and I probably only scratched the surface today by limiting my research to the Teen of Steel's Adventure Comics stories. In order, then...
Source: Superboy vol.1 #14 (1949) and Adventure Comics #195 (1953)
Sutri is a decidedly not-green boy on Mars who gets the Superboy Powers Suite when he's exposed to a red meteor. He becomes the planet's greatest hero. It turns out that these Martians' ancestors escaped from the doomed continent of Mu on Earth, and that now, Mars is doomed too, unless Marsboy can get a secret formula out of the Sphinx in Egypt. Superboy tries to stop him from stealing a landmark, and so begins Marsboy's big plan to keep Superboy away from him, first by becoming a bespectacled student at Smallville High to learn Superboy's secret identity and weaknesses, and then by making Superboy radioactive so he can't follow Marsboy around without poisoning innocent bystanders. It ends with Superboy escaping the trap and getting the formula out of the Sphinx thanks to a small difference in his power set (he can see through copper, while Marsboy can't) and Mars is saved. Marsboy returned for a Lana Lang jealousy trap story, in which poor Sutri was forced to do her bidding, a ploy so transparent, Superboy immediately started dating a Martian girl just to piss Lana off.
Analog: Kral, "the Second Superboy"
Source: Adventure Comics #205 (1954)
A rocket crashes on a familiar Smallville farmhouse, and it contains a telepathic teenager from Titan (misattributed to be one of Jupiter's moons) rocketed to safety by his parents before the planetoid could explode. There are all sorts of wrong with this picture, but the Kents adopt the boy Kral. It's all a lie. He's actually a scout for the war-like Titanians, here to find out all he can about our weapons. Life on Earth is so good and sweet, that it keeps melting his heart. In the end, he still resolves to bring Superboy to Titan under false pretense and entomb him in kryptonite, leaving Earth undefended. It backfires though, and the people of Titan are afraid we're ALL like Superboy, so they chicken out of the invasion, leaving Kral with a mission to make Titan a better place to live. (So did they remember this boy's telepathy when they created Saturn Girl some years later? Of course, Kral's got other powers, like emitting heat from his body to melt criminals. Is Saturn Girl actually the first Legionnaire to get nerfed?)
Analog: Joe Smith, Man of Steel
Source: Adventure Comics #233 (1957)
The radiations of a meteor give Joe Smith super-powers, and Superboy must prevent him from being exploited by a greedy promoter. And it's the story of how a random dude stole Kal-El's nickname years before he would have need of it. Joe's a bit of a bungler, so he causes trouble whenever he uses his powers, so it takes Superboy to clean up after him. To get him out of a contract with the promoter, Superboy follows an elaborate plan to make it look like Joe's lost his powers (it involves, for example, lying in the earth and holding the roots of a tree to it can't be ripped out of the ground). They might have just waited, because the powers only lasted a day anyway. Joe Smith needs to come back with his lawyers though.
Analog: Krypton Kid
Source: Adventure Comics #242 (1957)
Zar-Al is a Kryptonian teen sent by his father in a time-ship to Earth in the future in order to find a supply of Zeelium, the only element that can dampen Krypton’s uranium core. Didn't know Earth was rich in Zeelium? Join the club. Actually, we really don't have any (they look real hard, with giant shovels and everything), so Krypton is doomed. At least there was another Kryptonian scientist who believed Jor-El. Having failed in his mission, the Krypton Kid returns to his place and time to meet his fate with his family.
Analog: Allen Kent
Source: Adventure Comics #260 (1959)
While Superboy is away in space on a mission, the Kents adopt a boy from Smallville Orphanage for a month, who turns out to be a super-powered alien named Vidal who takes Superboy’s place in his absence. Though he has his own costume, and wears it around the house, he always disguises himself as Superboy when doing good deeds, but is a bit of a blunderer when it comes to keeping his secret identity safe. Good thing the Kents are on the job. The truth? His people have been keeping Superboy busy in outer space deliberately while Vidal finds Superboy some good parents (it must be lonely being an orphan, you know), using his powers of suggestion to get families to adopt him so he can test their ability to keep a super-teen out of trouble. The Kents are so good at it, Vidal leaves telling Superboy not to let his foster brother Clark know about his super-identity. Haha.
Source: Adventure Comics #287-288 (1961)
Before Krypton's destruction, Dev-Em was a juvenile delinquent always pulling destructive pranks on his neighbor, Jor-El. He's a clever boy though, and he believes Jor-El's warnings and outfits a shelter to take him and his folks to another world when the planet blows up. He awakens on Earth after years of suspended animation, learns that Jor-El's son is Superboy, sends him to the Phantom Zone and impersonates the Teen of Steel, ruining his reputation. He then releases Superboy and takes his still sleeping family to the future. Superboy is asked to leave Earth because he's been such a jerk, but Smallville's police chief talks the pitchfork-waving population down by "proving" Superboy was under the influence of Red Kryptonite. Dev-Em sounds familiar? That's because he finally settled in the 30th century where he turned over a new leaf working in the Inter-Stellar Counter-Intelligence Corps after declining Legion membership (in Adventure Comics #320, 1966). Not quite on par with Mon-El, but still a memorable recurring character.
Analog: Prince Mark
Source: Adventure Comics #303 (1962)
Prince Mark of Sardonia, marked for death by the evil Lord Hawke, is not only Superboy's physical double, but he wears a big-ass "S" on his shirt and a cape too. It's a chance to play Prince and Pauper for Kal-El, and lay a "super-kiss" on a princess. His cover's blown by a crown with an acid-released Kryptonite compartment, but Superboy still manages to defeat the usurper and his Kryptonite-bearing trained hawks.
Source: Adventure Comics #304 (1963)
Oh that Em family... I don't know how Roz-Em is related to Dev-Em, but he's a no-good criminal too, AND a body double for Jor-El. He was imprisoned in an orbital satellite by Jor-El (crime-busting scientist), but has now escaped. His plan? To impersonate a time-tossed Superman and tell Superboy to go packing and get off the planet. Roz-Em is rumbled with the help of the Kents and Krypto (good dog!) and is sent to the Phantom Zone where he eventually became one of Jax-Ur's cronies.
Now, I'm sure there are more, since Superboy's adventures weren't limited to Adventure Comics. Any favorites you want me to tackle in an eventual sequel?