There've been two anime I could not possibly give a bad score so far, but for different reasons.
The first was what is now my favorite anime of all time,
Sora yori mo T?i Basho / A Place Further Than The Universe, because it hit me so hard emotionally I didn't care about anything else that may have detracted from it.
The second one is another one entirely,
Sh?wa Genroku Rakugo Shinj?. I liked it a lot, but it was by far not my favorite anime of all time. That being said, I couldn't fault it. By which I mean, there was nothing, and I do mean nothing, I could critique about it. It was just... objectively too good.