Grammar? Punctuation? Artistry?

Hey friends, it's another chance for me to pick your brain on a random topic. This time, I'm wondering about grammar rules and your opinions.

Now, I know this seems general, and that's because it is. I'm a fierce grammarian and spelling is my FAVORITE (seriously) but I know it can all get a bit... annoying. And rigid. And overdone. I mean, I understand that there are rules that need to be followed, but seriously- what about artistic license? Sometimes saying things incorrectly is more dramatic, and therefore part of your own way of writing.

So what I'm asking is this- what are the grammar rules that you believe CANNOT be broken, and why? What rules do you flout with a smile? Where do you draw the line between artistic license and lunacy?

Short post, I know- I want some feedback, people!

Happy Reading!