So let's break this down folks: The Tesseract is considered to be a source of UNLIMITED power. Mar-Vell made a light speed engine using the power of the Tesseract. Said engine blew up in Carol's face! Thus granting her... unlimited power!
Now other heroes have been infected with Infinity Stones before. Vision's got one in his head, Dr. Strange wears one on his neck, Scarlet Witch is infused with one, Jane Foster and Malekith were infected by another, and Star-Lord and Ronan held one!
But the one Carol got laced into her was considered the crown jewel of Odin's vault. It destroyed the Red Skull and brought Loki's army to New York. The Tesseract has been the most unpredictable of the Infinity Stones. It's basically the equivalent to a super star in Mario!
Now, that's not to say that she's going to show up and one-punch Thanos. After all, he did crush the Tesseract with his bare hands, and he's got it's power, plus the 5 other stones on his gauntlet. So it's sure to be a fair fight. Let's just hope the Avengers can pull it off!
But until that time, looking at the facts, Captain Marvel is the most powerful her in the MCU!