What is the best anime with 13 or less episodes?

This has to be a question that has been asked many times, but I want to ask you guys about it because you are the best. I just finished the whole Dragonball Z anime and it took me 3 months. I want something that I can finish in 2 days. I don't care if it's a sad or depressing anime or something happy and uplifting. I just want your favorite anime that has 13 or less episodes.