I was all Marvel, thanks to the X-Men cartoon, and my local library for having Marvel Masterworks. I became a loyal Marvel reader, as DC heroes were lame, right? Then I discovered Kyle Rayner, Wally West, and Connor Hawke, and began to appreciate what DC had to offer over my beloved X-Men and Fantastic Four.
Edit: When I was much younger than that I'd bought (or my accurately, my mother had) Transformers and GIJOE comics when I saw them over the years, but I don't really count that, as it never lead anywhere else.