[S]omeone who, regardless of intentions involved, exploits culturally sanctioned beliefs to put down and denigrate someone who belongs to a minority.
The worst kind of racist I know of: the one that says, because you don’t fit my stereotype of what an X is, I will compliment you by saying that you are not at all like what an X is. Rather, you are just like me, don’t you know.
I certainly agree that the majority of racist opinions and acts involve the marginalization and subjugation of members of smaller groups, but these are not the limits of racism. It's a truly multifaceted threat, rearing its ugly head whenever any individual embraces and/or perpetuates a negative stereotype against another. Also, while the type of latent, patronizing attitude John describes is undeniably hurtful and problematic, I'm not sure that it's emblematic of "the worst kind of racist." Members of this ignominious category use their biases as the basis for the infliction of more tangible persecution and pain upon others. I remember reading, as a shocked high school student, of the horrible murder of James Byrd, Jr. by three white supremacists in 1998. Those men, and others like them, are the worst kinds of racists.