Mary Sue
Originally a Mary Sue is a character that is a shameless self-insert, poorly developed, without flaws and stupidly overpowered. /tg/ hates Mary Sues.
Unfortunately, after so many shitstorms and trollthreads, /tg/'s definition of a Mary Sue is not as clear as it used to be, now generally including any poorly developed, flawless and stupidly overpowered character. Using the term in this context easily makes oldfags RAGE.