I'd agree.
Much as Smith's Conan is an artist tour-de-force, with outstanding R.E. Howard adaptations, Buscema refined what could be considered the definitive Conan.
Even an artist of Neal Adams' caliber, when he drew the character, conformed to the Buscema image of the character.
I'm disappointed over the last 20 years or so that another artist, if not surpassing Smith or Buscema, hasn't at least presented another memorable interpretation of the character. The last that really did it for me was Rafael Kayanan. In particular a sequel in SAVAGE SWORD aound issues 210-215 that was a sequel to "Red Nails". While an original, Kayanan's art was also visually consistent with Smith's "Red Nails". There was a magazine called CONAN THE SAVAGE that followed SAVAGE SWORD's cancellation that presented a number of artists in one-shot fashion, most of whom could have done a memorable run.
Buscema illustrated CONAN THE BARBARIAN and SAVAGE SWORD for about 15 years, certainly plenty of time to leave a memorable imprint on the character.
