I picked the Weasleys. No matter what, you couldn't do without Ron, Ginny, Mrs.Weasley... but JKR could have left out the Malfoys and the books, I think, would barely be affected. Make one of the other Slytherins his enemy... have someone else lie to Voldemort... take the trio somewhere else when they were caught by Snatchers.
Honestly, the books would be another story without the Weasleys, but the Malfoys weren't very important.