I had to shell out a 9 here, because as beautiful as Kanon was, the few flaws remained, and as such it isn't quite "perfect". Still, overall this was a very enjoyable and addictive series.
Being a KyoAni series, it's all but unimportant that a suitably high quality animation be sustained. To be honest, some scenes could have been portrayed better. The eyeshadows from episode 18 were a good case of this - there's better ways to set up the required mood for that episode, such as covering their faces with hair, which they've done before in this series.
For voice acting, there was one scene in Shiori's arc (by, ironically, Kaori's seiyuu) which I didn't feel quite came across like I'd expected. After the conclusion to her arc, however, I realised this was probably intentional. The performance by each and every seiyuu has been simply breathtaking - from the energetic Ayu, to the remorseful Shiori, to the taciturn Mai, and so on - every character was portrayed accurately and beautifully.
My main gripe is with the pacing of this entire series. It's like a rollercoaster: For the first 10 episodes, we're going up the ramp. Then Mai's comes, and we plummet to doom. Shiori's story is another uphill rollercoaster ride, followed by the not-so-epic final sector with Ayu and Nayuki. At this point the 'coaster starts speeding beyond safety limits, and unharmonically slides off the tracks. That's by no means a representation of my thoughts about each arc; just how quickly it flashed by.
Everything about the series was beautiful, but the nature of the plot definitely detracts from the overall experience. At Makoto's arc, I was bawling my eyes out. With Mai, it was down to normal crying. Shiori had to make do with sniffles, and by Ayu's turn, I didn't have any emotions left. Even the events of episodes 22-23 didn't manage to do much more than make me think, "Oh, so that's what happened..."
Overall, the series would have been perfect if not for the sloopy pacing and deviant-based plot. Still, it was a very enjoyable experience, and it's successfully left me hanging for more. Bring on Clannad!