Wait, let us explain!
Thanos is not like the other MCU villains on this list. He wasn't underdeveloped or overexposed; he wasn't thinly written or completely forgotten about. And yet, audiences absolutely hated him - and for good reason.
Thanos was the original big bad of the MCU, making cameos and appearances along the way until he began to play a more pivotal role from Guardians of the Galaxy onwards. He was the threat that had been looming from the very beginning, with his sole desire being to collect all of the Infinity Stones and wipe out half of the universe. Except no one expected him to actually accomplish that goal.
The Avengers-level threat was so powerful that he only went and succeeded in his mission, killing half of the universe, including so many beloved Avengers. The moment left fans distraught, heartbroken, and seething, and Thanos instantly became public enemy no. 1. The difference between him and the others on this list is that fans loved to hate him, because he was an excellent villain executed to perfection.
The Avengers may have saved the day but not before enacting a plan that took both Black Widow and Tony Stark from us all, so forgiveness was never going to be on the cards for Thanos.
He'll always be a great villain, but he'll always be a loathed one, too.