Edo Madara easily.
Hiruzen was only meant to be the strongest on Part 1 of the series, now that the manga has progressed Hiruzen doesn't even male my top 20 list.
He is the weakest Hokage out of the 5 IMHO.
Unless Kishi shows some of Hiruzen's infamous "Prime" he will stay that way.
Most of the shinobi who hyped up his prime were not even alive to witness it, such as Kabuto and Iruka.
I would take Hiruzen's prime into serious consideratiOn if someone like Hanzo, Sannin, Onoki, Raikage, Bee, Kakazu, or Tobi were the ones hyping him up because their words have more credibility than weak ass Iruka and a Part 1 Kabuto.
The current Kages have yet to even mention Hiruzen let alone acknowledge his power in his prime in which Onoki Raikage and Tsunade should know all about.