It may be difficult for anyone not a singer to answer this definitively, but I would say it would depend on the singer as much as the role. Lilli Lehmann famously said she would rather sing all three Brunnhildes than one Norma, but I suspect Joan Sutherland would have felt differently. Either part might be a candidate for most difficult female role, with Isolde very much in the running. For tenors, Tannhauser, the Siegfried Siegfried, and Tristan are all notorious killers, but I imagine Otello is a close challenger. Staying with Wagner, the Walkure Wotan requires just about everything, vocally and dramatically, a bass-baritone can give.