I'm not against Mario necessarily, I am, however, against Nintendo and the mean things they did to Sega back in the 80s. Now, if you could just do an, "I support U2 and Coldplay," stamp, you'd have yourself some controversy.
There's also the Vocaloid vs Touhou war. But the only reason they're enemies is because of their music. Touhou's a doujin danmaku game while Vocaloid is a voice synthesiser. Both have excellent music.