Why Scarlet Witch is the most powerful avenger
Scarlet Witch is one of the newer heroes introduced in the later films of Avengers. She can move things without touching them and read minds. She first appeared in Captain America: Winter Soldier as part of a H.Y.D.R.A experiment along with her twin brother Pietro (Quicksilver). Her powers were a result of being exposed with the infinity stone.
In the Avengers: Infinity War, we can see that she is actually a competent fighter when they were being attacked by the Black Order. In Wakanda, the Proxima Midnight had to halt and wait for her to get out of the tower to take the stone out of Vision. They knew she was a huge threat and a difficult opponent to deal with. She even managed to kill Proxima Midnight, hold off Thanos while he is using several infinity stones, killed her lover and destroyed an infinity stone.
Her True Origins
In MCU films, Scarlet Witch is from Sokovia and her twin brother is Pietro. Both of them agreed to be part of the HYDRA experiment, promising new abilities in order to protect their homeland. They see the Avengers as a threat, the experiment failed and several participants died except for the twins. Being exposed with an infinity stone gave them abilities. With Pietro being able to move swiftly and her being able to use telekinesis and telepathy.
In the comics, Scarlet Witch and Quicksilver is the daughter of one of the most famous XMEN character, Magneto. That makes them mutants. An Elder God altered her abilities, giving her access to chaos magic.
Scarlet Witch’s Real Powers
In the film, we can see her making objects move and getting into people’s minds. Her powers actually is being able to manipulate psionic energy, the red thing coming out of her hands whenever she uses her abilities. She can shoot and blast her enemies, disintegrate them (like the one we saw in Age of Ultron when she took down several of Ultron droids when she felt the death of her brother).
Aside from psionic energy manipulation, Scarlet Witch also has telekinesis. Which means she can mentally manipulate, move or levitate objects. Her telekinesis is even powerful enough to break through vibranium, the strongest metal in Earth.
Her other ability is being able to read minds, mentally communicate with them and even experience what they are feeling. She can see through their deepest fears. She can even alter their state of mind and literally manipulate them, not Professor X level but powerful enough to hold many people off. We saw her doing that in Avengers: Age of Ultron where she was able to manipulate Captain America, Iron Man, Thor and even making the Hulk go out.
What She is Capable Of
In the comics (House of M), she was able to strip 90% of the mutants of their abilities during a mental breakdown. Aside from that, she also has the abilities to warp reality. She was able to create an alternate reality where the Xmen and New Avenger’s desires come true. Especially her father Magneto who is now the supreme leader of all mutants.
Although not an immortal like Thor, Scarlet Witch is an enhanced mortal who would put up a good fight. Yes she can be killed when off guarded. We can see her sustaining injuries when she fought against Proxima Midnight. But her abilities and what she can do is almost limitless. She is the only avenger who was able to destroy an infinity stone. Making her one of the most powerful among the avengers.