A lot like Lightning above, I'm very stingy with giving perfect scores. If something gets a perfect score then that implies it's utterly flawless with no improvements possible, which to me just doesn't seem possible. But the anime that I'd rate highest is La Storia della Arcana Famiglia. It's a really involved anime with great characters that build up and interact wonderfully, alongside a very gripping storyline and great fight scenes (when they are actually put in!), just making it a wonderful anime overall. The concept of the arcana cards is very original and it absolutely fascinated me so that's probably what originally got me hooked, but to anyone who hasn't seen it I'd definitely recommend it.
The only other anime I can think of that came close was Sword Art: Online, but the resolution to Aincrad, well... I didn't take it well haha. Those who've seen the anime will know what I'm referring to, but after that moment I didn't bother to watch the second half since I just didn't think it would have the same feeling any more. I'd give the first half 10/10 bar the ending, but honestly I haven't tried watching the second half so I can't comment on that.