Well in the Superman/Batman series, Mxy and Bat-Mite destroyed many universes while just sparring, but the Superman/Batman series, if I remember correctly, are not in continuity, so I dont know......
But under Grant Morrison, every 5D Imp were said to be capable of physically destroying and remaking a universe with ease. Other than that, we saw what happened to Joker when he stole Mxy's powers. He was basically omnipotent. And I seem to remember that the 5D Imps arent really affected by any laws of creation since they exist outside of it.
But I hate that annoying, arrogant old geezer, so Im going for Phoenix.