My favourite is possibly Nancy Kress's (... fiction that replaces one or more facts about our current world with speculative element(s) that are presented in a way that does not seem magical. That element might be scientific or technological change, or sociological change, or just a time change -- a future reality instead of today's.), and I think Mamatas also approaches my own take when he says:
"Science fiction is that subset of fantasy fiction in which the rationalistic, as opposed to the romantic, inclinations of the audience are valorized by the narrative."
Though personally I would have stated it slightly differently if I were stating my own view:
"Science fiction is that form of fiction in which the romantic inclinations of the audience are valorized through an at least reasonably plausible rationalistic narrative."
I could go on ... and on ... but I'll spare your poor scarred brains any further injury.