I think even if Durant proves to be a Hall of Famer and Oden never stays healthy, it still wouldn't be the worst draft pick in franchise history, or a contender, for one reason: the pick made plenty of sense at the time.
It may be the most unfortunate pick, the unluckiest pick, but not the worst, because it wasn't a silly or dumb pick. The Bowie pick honestly didn't make sense at the time, because it was made from an illogical basis: select need over talent. Even at the time, very very few people thought Bowie was as or more talented than Jordan. He was simply one of the top big men and the Blazers were bound and determined to take a big man. When you're trying to maximize your chances of getting a top player, passing on a superior talent is always a mistake.
Similar things could be said about Webster. In Nash's view, the team already had its "point guard of the future" in Telfair, so he wasn't going to take another one. Disqualifying superior talents in Paul and Williams, due to perceived positional needs, fit right into the flawed basis that characterized the Bowie selection.
When Portland chose Oden, it wasn't a need over talent selection. At the time, the team believed it had a franchise perimeter player and big man in Roy and Aldridge. So it's unlikely that they felt they had to choose based on need. In addition, the consensus was that Oden was the most talented prospect in addition to being a big man. Oden had been considered a phenom since he was a sophomore in high school. His collegiate performance didn't contradict those scouting beliefs. While there will always be nay-sayers about any prospect (I remember people who were even detractors of LeBron James, when he was still just a prospect), Oden had a consensus about his ability that was near those for LeBron James and Tim Duncan, from my recollection.
So, it was the right pick based on the information at the time. That disqualifies it, in my mind, from being the "worst" pick (which, to me, connotes ineptness, short-sightedness or horrific scouting). Oden's performance when healthy has validated the scouting. The injuries, IMO, were not predictable. So if he remains injury-prone and busts for that reason, I'll chalk it up as supreme misfortune, not a terrible pick.