Reaper. No contest.
Even in Rondo, which one one of the easiest games in the series, Reaper was a problem. And I am not talking about battle with him in Dracula X. If my memory serves me right, he was weak only in three games - HOD, Legends and SOTN.
While Dracula was weak boss in most if his appearances save for COTM, CV3, Dracula X (more or less), CV64 / LOD (as Drago and Ultimate Dracula), and arguably OOE.