The Infinity Gauntlet might be Thanos' most iconic weapon, but when the Mad Titan wielded the Heart of the Universe, he's never had a stronger power. In Marvel: The End by Marvel Comics, Thanos takes on an omnipotent ability that dwarfs any power he previously obtained. After absorbing the Heart of the Universe, Thanos becomes connected with everyone and everything, leading him to destroy a dying universe and build something new out of its ashes.
Thanos has gone out of his way to obtain some of the Marvel Universe's greatest powers on numerous occasions, from the Cosmic Cube to the Infinity Stones that power the Infinity Gauntlet. The Mad Titan enjoys making a grand statement with his incredible weapons, most famously wiping out half of the existence to please the living embodiment of Death during the Infinity Gauntlet saga. However, Thanos once gained powers called the Heart of the Universe, which was so strong, that after they were used to rewrite the entire Multiverse, Marvel Comics themselves had to reiterate it wasn't canon.
In Marvel: The End by Jim Starlin, Al Milgrom, Christie Scheele, Heroic Age, and Dave Sharpe, Thanos obtains the great power of the Heart of the Universe. Thanos quickly realizes the energy, described as usurped supreme power, is stronger and allows him to manipulate reality even more than the Infinity Gauntlet or the Cosmic Cube. As a result, Thanos becomes omnipotent himself. Thanos even admits the power brings him closer to god-good than ever before.
What Thanos does with the Heart of the Universe is nothing short of miraculous, as the power proves to be immediately more impressive than the Infinity Gauntlet. With the knowledge the universe is dying, Thanos unleashes his new power, killing Celestials and other cosmic-powered gods, including the Living Tribunal. He ends up absorbing the entire Multiverse until nothing remains. After existing in a realm of nothingness, Thanos would use the Heart of the Universe to heal space and time, wielding his powers to remake a universe on the brink of death. While the Mad Titan has often been viewed as one of the most destructive forces in the universe, in the end, he sacrifices himself to save reality.
It's important to remember that the Heart of the Universe isn't canon, as Marvel: The End aims to tell a story about the hypothetical end of the universe. So, in canon, the Infinity Gauntlet might be the strongest power that Thanos' has ever wielded. But, outside of canon, no Marvel Comics hero or villain has ever been more powerful than when Thanos wielded the Heart of the Universe and saved the universe with it. It's his ultimate form for a reason.