I thought Mulholland was the hardest by a significant margin. I didn't like the flat, somewhat windy section in Heartbreak, but you will power through that with a smile on your face. The two best training rides in my opinion for KOM:
1) Angeles Crest (La Canada to Dawson's Saddle), which is steady, constant climbing with nothing too steep -- similar to Breathless Agony.
2) Any combo of Santa Monica canyon roads that adds up to 100 miles and includes Decker right in the middle -- basically just doing a MC practice run.