I'd say Thanos from the Marvel universe is up there. With the Infinity Gauntlet, he could control reality and time. His intelligence and strategic thinking also make him a force to be reckoned with.
I think Sauron from The Lord of the Rings is a strong contender. His dark magic and control over vast armies make him a truly formidable fictional villain.
It depends on the specific fictional universe. In some settings, Q might be considered the most powerful, but in others, there could be characters with even greater abilities.
I think it could be Goku from Dragon Ball. His ability to constantly grow stronger, achieve new forms, and his mastery of powerful energy attacks make him one of the most powerful fictional characters. Plus, his never-give-up attitude adds to his power.