Warning: contains spoilers for Silver Surfer Rebirth #2!
The single strongest Infinity Stone is actually the weakest in the Marvel Cinematic Universe - and Thanos has just lost it. The Mad Titan is strongly associated with his conquest of the six Infinity Stones (called gems in the comics) and has not hesitated to use one or all to kill anyone in his path to Godhood (or his path to court Death, depending on the source material). But Silver Surfer Rebirth #2 plainly states that one Stone is more powerful than the other five - and it's not the all-powerful Time Stone.
Norrin Radd, the Silver Surfer, speeds through the cosmos on his board, now free from the grip of Galactus, his former master (the event was chronicled in the famed Galactus Trilogy of Fantastic Four fame). The Surfer suddenly finds himself in a battle alongside Mar-Vell - a hero who has long since passed. The Surfer is puzzled but has no time to think, for a bright flash of light brings him to a mysterious planet and face-to-face with Thanos himself.
A fight begins and the reader can see that Thanos and the Surfer - even while the latter is imbued with the Power Cosmic - are relatively evenly-matched. But Thanos doesn't seek the Surfer's death, but rather his aid. "Does it trouble you? Do you perhaps question reality itself?" asks Thanos, before revealing that the Surfer has every right to question reality: the Reality Stone has been stolen. "The most powerful of them all, the Reality Gem, [Adam Warlock] entrusted to me."
In the MCU, the Reality Stone (at least by itself) is little more than a glorified illusion device. Thanos can disassemble Drax and Mantis and show Doctor Strange a vision of Titan in the planet's prime, but the effects are temporary. In the comics, however, the Reality Gem can change the fundamental laws of the universe, such as gravity or thermodynamics, at the user's will. It can allow the user to travel from one reality to another or even create their own (if its power is boosted with the addition of the other five Gems). Only supremely higher beings such as the Living Tribunal would be aware of the manipulation and would be able to withstand its power if they saw fit to do so.
In general, the Infinity Stones of the films are not as powerful as those in the comics (and notably, Thanos did not have to close his fist to activate their power; he could simply control the Stones through his mind alone). The Reality Stone is indeed the strongest of the six, provided the user has a clear understanding of their desires and laws of the universe. Unfortunately, now that Thanos has lost the Reality Gem, someone else without his restraint or planning could wreak untold devastation in the Marvel Universe.