Mine is that Native Instruments plugin called Razor. It’s an additive synth that devours your CPU without mercy. The worst part is that it’s not even that high of a quality synth, compared to similar CPU hogs like Omnisphere or Diva.
Actually, gotta be honest with you, u-he Diva and Dmitry Sches’ synth Diversion win my vote. They are CPU hoarders, and I vote them each about the same as far as usage (with the assumption that the oversampling is at it’s highest setting when using Diversion).