Your words: "To my knowledge - all Tobirama did was aid = not even major role = Hashi with creation of leaf...Create the Edo - which is causing endless problems this far into the series -"
Aiding in the creation of the Leaf during the age of war is not an easy feat; it's a major accomplishment.
Creating the Edo, which is arguably one of the greatest techniques in the discernible Narutoverse, is an exceptionable accomplishment. Moreover, it is "causing endless problems" because it's a great technique in which 2 major antagonists- Kabuto and Orochimaru- used to wage total war on Konoha and the entire ninja world. Pretty major, huh?
Also, Kishimoto has not provided enough information about Tobirama to make such an assertion. Therefore, the clear answer to "Is Tobirama the weakest hokage" should mirror choice D on a multiple choice test- "not enough information."