That’s the million dollar question isn’t it? I think the most used name now is Black Reaper or Yamineki.
Akaneki was the first name given to him because originally the most recent Kaneki’s hair was dark because it was coated in blood (I think it grew out and he trimmed off the white part post battle).
I’ve forgotten where the name Yamineki originated from, but I think it’s something like dark, demon, or even shadow Kaneki.
A lot of people unless they are differentiating will call him Kuroneki, or just Kaneki or Haise.
I’d say though if you want to make absoulutely sure people know who your talking about say “Black Reaper Kaneki”