"a type of character who is depicted as unrealistically lacking in flaws or weaknesses."

Powerful characters aren't instantly bad there are many broken and interesting characters, Aizen, Bill Cipher, Madara and ETC a Gary Stu is cough:

Chad/Half-Devil Half-Angel/ Son of Satan/ Son of God/ True First Super Saiyan/ Mastered Ultra Instinct/ immortal/ Single

Basically a character that has no flaws uninteresting and shitty