9/10 - KyoAni version (the Toei version doesn't even seem worth rating, to me)
For a eroge made anime, this was quite amazing, IMHO. It would have been so easy to make this into a standard male-fantasy harem anime, with LOTS of fanservice and drooling fanboys and little in the way of story, but KyoAni stayed true to their vision of Key's original story and made basically a series of very touching stories woven together with real meaning. The little fanservice that was present was interleaved into the story well enough to seem natural in context (I mean, imagine seeing a girl naked in a bath, <gasp>!
).
Many stories go for the cheap win by dragging out a pretty girl and putting her through hell without letting us start to care about the actual character. But Kanon avoids the narm by allowing us time to get to know the characters first, so each individual arc's conclusion comes with a lot of emotional investment. Then, at the last, the whole series comes together with a surprising heartwarming ending in a series so replete with sadness and tragedy.
Ultimately it was the constant intersection between heartbreaking and heartwarming that made this series for me. The end of Episode 10 can get me fighting back the tears every time I watch it, no contest.
BTW - Apparently, almost every one of my favorite series has Maaya Sakomoto's voice in it at one point or another.
Ciao!