Humm, I wouldn't say that... Zangetsu never said "You have to find the bankai weapon". He simply said something along the lines of "in order to achieve bankai you have to find your soul slayer and strike me with it, since it's the only one that can actually hurt me"... So the sword he used is the one he has always used, the bankai form is simply a form of that weapon, not a different one...

At least that's how I remember it... might be wrong since I don't remember much about the exchange of words when they were trying to achieve bankai....

So I don't see why his usual sword would seem as the "weakest" unless you are meaning that the one that looked as the "weekest" is the sword in its bankai form, which as I said above, I don't think is the case of what happened.