Since @Tyrannus did the comics version, I'll do the MCU version, and the reasons why of course (and do the best wielder, not guardian, for all of them).
1) Reality (f with reality, like you're Scarlet Witch with a lantern ring)
2) Time (screw with time however you'd like pretty much; risky to use but not much skill required; more like intelligence)
3) Space (manipulation, alteration, and transmission through space; easy to wield)
4) Power (allegedly infinite power, but really only big purple blasts and passive strength and durability when not glowing; tends to painfully disintegrate those touching it)
5) Mind (tends to mess with the users head, can shoot powerful beams and control victims minds but only on contact it seems)
6) Soul (not much practical alone, use especially when it comes to combat)
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1) Reality - Prof. Hulk/Tony Stark: They're super genius inventors and slightly more stable than Rocket. The reason I chose Nerd Hulk over Banner is because Banner is not stable in the slightest. Of course, he could control the Hulk and all...but the temptation to erase Hulk would be too great, and he possibly would screw things up big time.
2) Time - Vision: Supercomputers are less likely to make mistakes.
3) Space - Watcher Informant: He clearly would make good use of it, whether traveling the universe or protecting himself from deadly aliens.
4) Power - Hulk: The best in every physical category pretty much, Hulk doesn't even need theoretical unlimited power when he already has it. Yet he'd still be the only character in the MCU to be able to successfully wield it alone indefinitely, without something to harness its overwhelming power. The more the stone would hurt him, the more he'd grow in his own power, which would mean infinite + infinite power.
5) Mind - Black Widow: If the Mind stone enhanced intelligence this would be a different story. However, when Natasha was an assassin, she'd easily control her subjects/victims minds to do what she wanted/tell her what she needed to know. And she'd be decent at resisting the manipulation due to her training. Plus, those laser blasts don't hurt.
6) Soul - Anyone with dead friends/relatives so that they could talk to them.
1. Reality - You can literally bend reality to whatever you want, however you want it. Your only limit is your imagination.
2. Time - If anyone defeats or prevents you from doing anything just go forwards of back until you get what you want.
3. Mind - In theory you have to actual power but you can control others who do have what you want.
4. Space - You can go wherever you want but that's it. This is where the stones go downhill.
5. Power - Infinite Strength but what good it that against the other stones?
6. Soul - Lol who cares about your soul in a fight?
1. Reality - Iron Man used it during the non-canon Contest of Champions (I think). He made it so he won the Civil War and kept humanity safe. I trust Iron Man.
2. Time - The Sentry has never used it however he'd be a good gatekeeper. Whenever the timeline goes wrong he can go forwards or back in time to fix things. He's pretty much the most OP hero in history and his atoms exist a fraction of a second ahead of the current timeline anyway so he'd be durable enough to survive potential threats.
3. Mind - Charles Xavier wouldn't need Cerebra anymore and he'd be able to use it to save mutants. He already imposes several rules on himself so he wouldn't use it for bad.
4. Space - The Hulk has always said he just wants to be left alone. However if ever he's needed, the stone will bring him back.
5. Power - Thing out of all the powerhouses in Marvel has the most heart. He willpower was stronger than the likes of Hulk, Thor and the others as credited by the Champion. He's already a nice guy so he wouldn't abuse his power.
6. Soul - Adam Warlock's already linked to the stone and literally spent time inside it. Who else?
1) Reality (f with reality, like you're Scarlet Witch with a lantern ring)
2) Time (screw with time however you'd like pretty much; risky to use but not much skill required; more like intelligence)
3) Space (manipulation, alteration, and transmission through space; easy to wield)
4) Power (allegedly infinite power, but really only big purple blasts and passive strength and durability when not glowing; tends to painfully disintegrate those touching it)
5) Mind (tends to mess with the users head, can shoot powerful beams and control victims minds but only on contact it seems)
6) Soul (not much practical alone, use especially when it comes to combat)
_
1) Reality - Prof. Hulk/Tony Stark: They're super genius inventors and slightly more stable than Rocket. The reason I chose Nerd Hulk over Banner is because Banner is not stable in the slightest. Of course, he could control the Hulk and all...but the temptation to erase Hulk would be too great, and he possibly would screw things up big time.
2) Time - Vision: Supercomputers are less likely to make mistakes.
3) Space - Watcher Informant: He clearly would make good use of it, whether traveling the universe or protecting himself from deadly aliens.
4) Power - Hulk: The best in every physical category pretty much, Hulk doesn't even need theoretical unlimited power when he already has it. Yet he'd still be the only character in the MCU to be able to successfully wield it alone indefinitely, without something to harness its overwhelming power. The more the stone would hurt him, the more he'd grow in his own power, which would mean infinite + infinite power.
5) Mind - Black Widow: If the Mind stone enhanced intelligence this would be a different story. However, when Natasha was an assassin, she'd easily control her subjects/victims minds to do what she wanted/tell her what she needed to know. And she'd be decent at resisting the manipulation due to her training. Plus, those laser blasts don't hurt.
6) Soul - Anyone with dead friends/relatives so that they could talk to them.
That's all folks!
2. Time - If anyone defeats or prevents you from doing anything just go forwards of back until you get what you want.
3. Mind - In theory you have to actual power but you can control others who do have what you want.
4. Space - You can go wherever you want but that's it. This is where the stones go downhill.
5. Power - Infinite Strength but what good it that against the other stones?
6. Soul - Lol who cares about your soul in a fight?
1. Reality - Iron Man used it during the non-canon Contest of Champions (I think). He made it so he won the Civil War and kept humanity safe. I trust Iron Man.
2. Time - The Sentry has never used it however he'd be a good gatekeeper. Whenever the timeline goes wrong he can go forwards or back in time to fix things. He's pretty much the most OP hero in history and his atoms exist a fraction of a second ahead of the current timeline anyway so he'd be durable enough to survive potential threats.
3. Mind - Charles Xavier wouldn't need Cerebra anymore and he'd be able to use it to save mutants. He already imposes several rules on himself so he wouldn't use it for bad.
4. Space - The Hulk has always said he just wants to be left alone. However if ever he's needed, the stone will bring him back.
5. Power - Thing out of all the powerhouses in Marvel has the most heart. He willpower was stronger than the likes of Hulk, Thor and the others as credited by the Champion. He's already a nice guy so he wouldn't abuse his power.
6. Soul - Adam Warlock's already linked to the stone and literally spent time inside it. Who else?