I'm probably in the minority, but for me it's Stegman. One could maybe blame Joffrey's psychopathy on his inbred parentage; Stegman has no such excuse, plus he's a rapist.

Stegman was spoiled too. Notice how his mother defended him when Dean Norris tried to enter his apartment.

Both were spoiled by their mothers, but Joffrey's parents are literally twin siblings. Being inbred can actually cause serious mental illnesses like bipolar disorder or schizophrenia. It's a miracle Myrcella and Tommen didn't turn out the same way TBH.