Your favorite GPT initiators that have actual usefulness

For example, I made these:

my personal language learning prompt
therapist prompt
flirt simulator prompt (beta)

I could imagine a prompt for learning the basics of anything so you just put in one sentence of the topic + details and it gives you a basic background, usecase, detailed knowledge dive after introduction and questions

But I think there I much more people are using it for. Maybe Lawyer, contract checker, whatever.

Any useful cases you love?