Spotlight
Players come to your game to have epic moments, to be the stars of the show. Some GMs and players talk about balancing character powers and capability, but I'd argue getting each player some time in the spotlight is much more important.
The TL;DR
You can't balance, so don't try
Avengers - all awesome, all different, each with moments in the spotlight
Offer the spotlight
The story is your spotlight
Imbalance is right
A wizard tearing apart the fabric of the universe to throw fireballs, a rogue moving through the shadows to bypass the guards, a fighter who can lop the head off the dragon and a cleric whose faith can raise the dead.
They're not the same, and don't need to be. Even if you look at things that have some general similarities like the damage output of a mage throwing a fireball vs a fighter swinging a sword if you did manage to balance these you'd wipe out the difference in the characters.
Players don't want to play the same character with the same abilities and capabilities as everyone else. They want to play someone who adds to the story in unique ways and has an impact where no one else could.
Avengers take the stage
My favourite example of this is the film Avengers Assemble - spoilers ahead (but come on you've had 7 years).
In the climactic scenes you have the Avengers trying to fight off Loki and the Chitauri. Involved in the battle you have two highly trained humans, an ancient god of thunder, a technical genius in a mechanical suit, a man transformed by his incredible anger and a 70 year old super soldier.
Each of them takes the stage for a moment in the spotlight, and those moments give each character screen time that makes them memorable:
Captain America - leads the team, bystanders in the bank and ordering the cops
Hawkeye - guiding the action and the 'no look' shots
Iron man - The confrontation with Loki and guiding the missile
Thor - Fight with Loki, lightning surges and taking down the leviathans
Hulk - puny god, tearing up the leviathans, catching Stark
Black Widow - riding the Chitauri, closing the portal
Black widow doesn't have the Hulk or Thor's strength and durability, but without her the portal wouldn't have been closed. Hawkeye couldn't fly the missile into the portal, but Hulk hardly has the intelligence to see patterns in the Chitauri attacks.
Each of them adds something unique to the story, that none of the others could, and the story is better because of it.
Offer the spotlight
During each session ideally, or at least in each adventure all your players should be offered a chance to step into the spotlight.
This can be a single roll that sees the player solve a problem that has stymied the party, an hour long fight in which they slay the most enemies or an in character negotiation where they bribe, threaten or barter a NPC to help them.
As a GM your job isn't to drag each character into the spotlight though. Some players prefer to sit back and watch the game, plan to take their character in a different direction or just don't want to engage with the scene you've set up. Don't worry about that, and don't force the moment, preparing the stage is your job, stepping into the spotlight is the player's.
The story is your spotlight
GMing isn't your chance to show off your amazing acting skills, inspire the players with your amazing rhetoric or read chapters of carefully crafted description. These skills are great to have, but shouldn't steal the focus from your players.
Use every tool at your disposal to make your game a better experience, but the camera's focus and the spotlight should be on the heroes, not the set design or the extras.
The spotlight you want comes after the game as players recount the adventures you helped them build, the diabolical villains they defeated and the heroic moments they made happen.
