There are six Infinity Stones which Thanos is desperate to get his hands on, and though only five have shown up on screen so far, the six will eventually be inserted into the Infinity Gauntlet by the big purple-blue guy, likely in the first Infinity War movie, granting him virtually unlimited power beyond anything seen in the MCU before.
But which of the stones have proven to be the most powerful so far? Let's take a look from weakest to strongest. In last place, there's the Mind Stone, which was given to Loki by Thanos to help during his invasion of Earth, inserted into his Scepter to help enslave anyone who got in his way (namely Hawkeye and Erik Selvig).
The Scepter later ended up in the possession of Wolfgang von Strucker, who used the Mind Stone's power to help unlock Scarlet Witch and Quicksilver's unique abilities, and after helping bring Vision to life, the stone was implanted in his head for safe-keeping, where it was capable of firing energy blasts.
Next up is the Power Stone, seen in Guardians Of The Galaxy, with Thanos charging Ronan with finding it, given its awesome potential to destroy planets. The Power Stone will also painfully disintegrate almost anyone who comes into contact with its pure form, and the only major way to prevent this is to share the energy among a group of individuals, just as Star-Lord and the Guardians did in order to destroy Ronan
Then there's the Reality Stone, appearing in Thor: The Dark World as the Aether, a red liquid which can be absorbed into an organic body, enhancing a host's strength and power (as in the case of Malekith), or sapping a human's life force and proving fatal without treatment (as with Jane Foster, who barely survives being infected with it).
Though Malekith's plan failed, it nevertheless proved the potential of the Aether, as he intended to use it during the Convergence of the Nine Realms, to plunge all of them into complete darkness. Yeah, that's pretty damn powerful.
And then there's the most iconic of the Infinity Stones, the Space Stone, better known as the Tesseract, which first appeared in Captain America: The First Avenger. The destruction it caused in both movies should alone show how powerful it is.
In addition to allowing the user to travel to any location in the known universe (Thanos used it to teleport Loki to the Tesseract in The Avengers), it has been used by Red Skull to create highly advanced weaponry, and Loki forced Dr. Selvig to make a portal for the Chitauri to get to Earth with the device. Heimdall also ended up using the Tesseract to fix the broken Bifrost bridge, so it's not all bad. Its power, though, is undeniable.
Most recently there's the Time Stone, introduced in Doctor Strange inside the Sorcerer Supreme's Eye of Agamotto. It allows the user to manipulate time itself on scales both small and large. It can rewind actions or accelerate them, lock people and places in time loops, and even undo major events, including death. We've seen glimpses of its power, but it should be a huge part of Avengers: Infinity War.
The Soul Stone could feasibly trump the lot, but it hasn't yet turned up - the safe betting is on it being on Wakanda, and the hunt for it forming a key part of Infinity War.
Of course, they're all nothing compared to when they're brought together in the Infinity Gauntlet, which is probably going to happen rather soon.
Which Marvel Cinematic Universe items do you believe to be the most powerful? Shout them out in the comments!