Just look at the title.
What is the best anime you have EVER seen and why?
Mine?
Gankustuou, based on the French classic, The Count of Monte Cristo. It's one of those weird but interesting vampire/revenge animes that keep you entertained for hours. The unique graphics and coloring make you all dizzy at first but you get used to it. There's even the best fight scene I've ever seen in this anime. I absolutely love it.
What's your favorite?