Good English
"Good" English does not exist. Only English exists. Spoken English and written English.*
"Good" and "bad" are either moral or aesthetic judgments. They are, therefore, subjective judgments. It's also possible to make objective judgments about English. For example, "This sentence is (un)grammatical". Sometimes, however, grammaticality varies by dialect, so a judgment about grammaticality is only a relatively objective judgment. "She's in hospital" is grammatical in British English, but not in American English, which requires "She's in the hospital". Grammatical English is not necessarily "good" or "bad"; neither is ungrammatical English necessarily "good" or "bad". All English is "good" or "bad" depending on context. Context includes the speaker or writer, the listener or reader (the audience), the purpose of the English, and the type of discourse in which it appears. "Good" English in a war novel may be "bad" English in a fairy tale or other type of children's story. Essentially, "good English" is English I like and "bad English" is English I don't like. Good and bad are in the eyes and ears of the beholder.
*(I would classify signed English (e.g., American Sign Language) as one type of written English because it must be seen to be understood. Braille must be touched to be understood, so it's another type of written English. Because I'm not fluent in either ASL or Braille, I don't want to deal with them. My comments here apply only to spoken and written English.)