My first would of been Legend of the Overfiend, when I was eight years old (my brother was a fantastic babysitter, his logic was probably "how bad could a cartoon be?) I then made a point of being illlogically scared of any cartoon that looked the remotest bit anime. Unless I had already seen it and knew it to be okay.
Thank god pokemon came along to help me get over that traumatic experience...
A side note is I watched Legend of the Overfiend again a few years back and have a completely different take on it, I still don't want to watch it again but this time because it was rubbish. The only nice comment I can give was that the music was okay.