DC Comics' Superman is without a doubt the most iconic superhero ever created, to the point that even DC's rival, Marvel Comics, took inspiration from the Man of Steel to create over the years no less than three characters based on the Last Son of Krypton. Obviously, none of them reached Superman's levels of popularity, nor his status as flagship character for the publisher, but their powers and abilities still put them among the strongest characters in the Marvel Universe.
Gladiator, Hyperion, and Sentry were created at very different times and for very different stories and contexts. While their powers look similar, they all have unique origins, which made fans debate about which one of Marvel's Superman "clones" is stronger, and if any of them could take on the original Man of Steel. This sort of comparison is always very subjective, but the three characters are different enough that it's possible to look at their origins and powers and then speculate who is the "best" Superman clone.
Gladiator
Kallark, aka Gladiator, is a faithful servant of the Shi'ar Empire from the planet Strontia, whose inhabitants train from a young age to obtain the coveted title of Gladiator, Praetor of the Imperial Guard. Gladiator's powers come from Strontian physiology, augmented with a technological process enhancement, and they make him one of the strongest characters in the Marvel Universe. Kallark has super strength, stamina, invulnerability, a healing factor, super speed, flight, heat vision, and freezing breath. However, he also has a major weakness. Strontian's powers are connected to their levels of confidence. If Kallark doubts himself, his powers are severely diminished, to the point that he can be beaten by a much weaker character. On the other hand, when he is fully confident, there are very few who can stand against Gladiator (usually just cosmic beings).
Gladiator was created by Chris Claremont for his legendary X-Men run. Claremont actually took inspiration not from Superman but from one of his supporting characters, Mon-El, a member of the Legion of Super-Heroes and a Daxamite alien who has the same powers as the Man of Steel (but a different weakness, lead). The legendary author used the Legion as inspiration when he created the Shi'ar Imperial Guard, but Kallark's name is actually a reference to Kal-El and Clark Kent, Superman's two names.
Hyperion
Of all the characters in this list, Hyperion (Mark Milton) is the one who fits the label of "Superman's clone" the most, and even his origin story is the same as that of the Man of Steel. Hyperion is the last survivor of an alien race (later retconned to be a branch of the Eternals), sent to Earth as a baby and raised as a human by a loving family who passed on to him a strong sense of morals that led him to become the greatest hero of his world (Earth-712). Hyperion and the other members of the Squadron Supreme were created by Roy Thomas and John Buscema to appear as guest villains/allies for the Avengers, and they were all based on DC Comics' Justice League. Later, these characters got their own titles and storylines, including a highly-recommended Squadron Supreme series written by Mark Gruenwald and penciled by Bob Hall.
There are several versions of Hyperion in the Multiverse, some (such as King Hyperion), stronger than others, but this article will refer to the original one (Earth-712). Hyperion's powers come from his Eternal physiology, but he has the unique trait of being able to collect cosmic radiation and turn it into energy with a process similar to nuclear fusion (mirroring Superman's reliance on solar radiation). He has the standard super strength, speed, resistance, and flight, and he is able to manipulate cosmic energy to a variety of effects, including "atomic vision" (heat rays projected from his eyes) and a regenerative healing factor. Hyperion has one weakness, and that's one substance called Argonite, his own version of Kryptonite.
The Sentry
Sentry is the most popular of the "Marvel Superman" trio but also the one who is most difficult to assess. His origins and powers went through several retcons, and actually the mystery behind Sentry's true nature is one of the major appeals of the character. Created by Paul Jenkins and Jae Lee for the limited series The Sentry, Robert Reynolds is an ordinary man who can transform into Sentry, the greatest hero no one ever heard about. Apparently, Sentry was the first and the greatest hero of the modern age, who was forced to make everyone forget about him to keep in check a dark, evil creature born from his powers, the Void. In truth, Bob Reynolds was a meth addict who stole an experimental enhanced version of the Super Soldier Serum, which gave him incredible powers but also exacerbated his precarious mental state, creating the Void as an expression of Bob's repressed psyche.
Sentry's powers are based on molecular manipulation or reality-warping, fueled by his altered physiology that is charged with solar energy, or "the power of a million exploding suns." So, the Sentry is not just strong or fast, he is whatever he wants to be. It has been theorized that the vast majority of his powers derive from his ability to alter matter and energy and that he could simulate almost any superpower with enough practice and control. However, Sentry's unstable mental state means that his powers can be unstable too, but he has demonstrated he can be one of the strongest beings in the Marvel Universe, especially after he finally merged with the Void becoming Dark Sentry, which gave him access to his full powers.
Who Is The Winner?
It's quite hard to pick a winner in this context, so it's best to start by picking who is the clearest loser: Hyperion. While he might be Marvel's homage to Superman, Mark Milton's powers are very far from those of the Man of Steel. Hyperion is "just" a particularly powerful Eternal, with a clear upper limit to his strength. Hyperion already fought against Gladiator and lost: the Strontian overpowered him and broke his back. The other two competitors, on the other hand, share the peculiarity of not having a definite upper limit for their powers, which depends on their mental state for both.
It could be speculated that a fully-confident Gladiator could take on the Sentry in his unstable state, but not when he merged with the Void and obtained full control over his powers. In that state, Dark Sentry's ability to manipulate matter and reality is simply more vast and flexible than Gladiator's straight-up physical powers. However, Gladiator's military discipline, training, and combat experience could still give him the victory, if he was able to maintain his confidence in front of the Sentry's absurd level of powers. In conclusion, Dark Sentry would beat Gladiator, but in any other case, the Praetor of the Imperial Guard would have the edge, even if by a small margin.
As always, fans will have their own favorites for this battle, and they can only hope that one day Marvel Comics will show an epic showdown between its three versions of DC Comics' Superman.