Other atheists. That's according to a new study co-authored by psychologist Will Gervais. Says Gervais:
I suspect that this stems from the prevalence of deeply entrenched pro-religious norms. Even in places that are currently quite overtly secular, people still seem to intuitively hold on to the believe that religion is a moral safeguard.
-Will Gervais, PhD
The scope of the study was international and according to the write up in The Guardian "Only in Finland and New Zealand … did the experiment not yield conclusive evidence of anti-atheist prejudice…"
From an American perspective this finding doesn't surprise me. I've met plenty of atheists who sort of believe that most religious leaders are closet atheists conning people out of their money. That kind of thinking reached fever pitch this year when Michael Shermer gloated about [white] evangelicals getting duped by immoral atheist Trump (I write why I think he's wrong here).
Some other circumstantial evidence comes from PRRI's 2013 American Values Survey. They found that the nones reported a score of 77 (out of 100) in a cold-warm scale. But that number dropped to 71 for atheists. Sample size limitations don't allow for an analysis of Atheists' responses.
Read the full Gervais et al. study at Nature Human Behavior.