My favorite way is still the old way, from before the advent of the crematorium. Dig out a massive cave, fill it with corpses, toss in a molotov or three.
But I kind of do all of the above. Sometimes I decide before I start that we're going the molotov route, sometimes the mass grave route, occasionally the butchery route... but usually I just use a crematorium. It's the one that requires the least planning.