When it comes to expressing the best or worst case scenario, there are a variety of synonyms you can use instead of the common "couldn't be better/worse/nicer etc." For example, instead of saying "couldn't be better," you could say "couldn't be more perfect," "couldn't be more ideal," or "couldn't be more excellent." On the other hand, instead of "couldn't be worse," you could say "couldn't be more terrible," "couldn't be more disastrous," or "couldn't be more dreadful." Similarly, instead of "couldn't be nicer," you could say "couldn't be more pleasant," "couldn't be more delightful," or "couldn't be more charming." Varying your language can bring more creativity and nuance to your writing or speech.