Izuna was said to be an equal with his brother Madara when they both attained the MS. So we can make a loose connection between MS Sasuke and MS Itachi, showing the level Izuna would of been around compared to Madara.
Tobirama was a pseudo Minato, had Edo Tensei and surpassed Kisame in water Ninjutsu.
Judging from these assumptions, i would have to give it to Tobirama.
Based on assumption, Tobirama. We know his skills regarding water style are unparalleled, combine this with S/T and edo tensei and you havr yourself a strong opponent? I'm just guessing here.