The MCU Phase 4 Confirmed That The Soul Stone Is The Strongest
Marvel's What If...? confirms that the Soul Stone is the strongest and most important of the six Infinity Stones in the MCU. Going beyond the reveals made in Avengers: Infinity War, the Soul Stone was a key focal point in the plan made by the Watcher's Guardians of the Multiverse (heroes summoned from various realities). It's certainly a logical conclusion, considering that the Soul Stone had more protection than the majority of the other Infinity Stones in the Marvel Cinematic Universe.
In comparison, the other Infinity Stones were not nearly as protected as the Soul Stone was. While most were certainly hidden such as the Space, Reality, and Power Stone (and presumedly the Mind Stone before Thanos found it), none had any kind of significant protection. The only exception beyond the Soul Stone itself was the Time Stone protected by Earth's formidable Sorcerer Supreme. Even then, only the Soul Stone demanded sacrifice and significant loss in order for it to be taken from the planet Vormir.
Related: Why The Soul Stone Is The Only Infinity Stone Without A Phase 4 Replacement
The idea 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 place among the Infinity Stones. You might say it holds a certain...wisdom". While this is an allusion to the Soul Stone having a degree of sentience in the original comics (before the Stones were reborn and all became sentient), it indicates the Soul Stone's greater strength above the others. As such, it makes sense that the Guardians of the Multiverse zeroed in on taking the Soul Stone from Ultron first in the finale episode of What If...?, correctly
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