Unfortunately, the abilities of the Soul Stone all alone were never seen in full in the MCU or in the alternate realities of What If…?, seeing as how Thanos obtained the other Stones rather rapidly. Nonetheless, the comics uncover that the Soul Stone allows its wielder to attack souls in a variety of ways, while also trapping souls in a pocket domain known as the Soul World seen momentarily in Infinity War. It also allows the wielder to control all forms of life regardless of sentience. While the powers and abilities granted by the other Stones are nothing to sneeze at, it makes sense that the Soul Stone would be protected on a greater level than the others given the sheer levels of impact and control it could give just all alone.
The simple fact that the Soul Stone was the most watched in the last two Avengers films communicates its greater importance. Furthermore, the Guardians of the Multiverse specifically pursuing the Soul Stone in What If…? further confirms its greater worth and strength. While the MCU has been slowly drifting away from the possibility of Infinity Stones as it continues into Phase 4 and then some, it would certainly be interesting to see the Soul Stone return in some fashion, albeit with a greater spotlight to match its greater strength and importance in the Marvel Cinematic Universe.
Marvel’s What If…? confirms that the Soul Stone is the strongest and most important of the six Infinity Stones in the MCU. Going past the reveals made in Avengers: Infinity War, the Soul Stone was a critical point of convergence in the arrangement made by the Watcher’s Guardians of the Multiverse (heroes summoned from various realities). It’s certainly an obvious end result, considering that the Soul Stone had more protection than the majority of the other Infinity Stones in the Marvel Cinematic Universe.
The possibility of the Soul Stone being the most important and strongest of the six fits quite well with the comments made by the Red Skull to Thanos and Gamora in Avengers: Infinity War: “Soul holds a special spot among the Infinity Stones. You might say it holds a certain…wisdom”. While this is an allusion to the Soul Stone having a level of sentience in the first comics (before the Stones were reawakened and all became sentient), it indicates the Soul Stone’s greater strength over the others. As such, it makes sense that the Guardians of the Multiverse focused in on taking the Soul Stone from Ultron first in the finale episode of What If…?, correctly remembering it as the Stone with the most importance. The fact that it both demands a special degree of sacrifice and is shown to debilitate Ultron so significantly after its evacuation highlights that the Soul Stone truly is the strongest Infinity Stone.