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.
A male Mary Sue can be referred to as a Gary Stu or Marty Stu, although this is very rare on /tg/.
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 character that the poster in question doesn't like, regardless of whether or not they actually fit the technical criteria. For example, anything that /tg/ thinks has even a hint of weeaboo is typically also described as a Mary Sue. Misuse of the term in this manner easily makes oldfags RAGE.