Almost certainly the worst band of the '70s was one that none of us have ever heard of. As others are ably pointing out, though, there were plenty of famous and successful '70s bands who were way worse, on almost any dimension you could name, than any of those listed, and many of those listed were, IMHO, damn good (and by any objective criterion of musicianship, just about all of them were).
I take the real intended question to be “Which of these bands best represents the worst excesses of Prog Rock?” (Although that does make the inclusion of The Rolling Stones a bit puzzling.) On that basis, I voted for Yes (though I can see a case to be made for ELP or The Moody Blues, and maybe Genesis). I am rather proud of the fact that I detested Yes, and for more-or-less the ‘right,’ punkish reasons (and despite the fact that many of my friends claimed to like them), long before punk arrived, and it became fashionable to detest prog rock. On the other hand, although I certainly saw the point of punk, when it happened, I still think that several of the bands on this list were way better than anything punk ever produced.
Also, is it not rather sad that people are still reenacting the trope of punk rebellion against prog over 30 years after the event? A good part of the punk critique of prog was that prog was old and played out, it was still rooted in the psychedelia of 10 years before, but had lost the youthful energy, anger, experimentalism and iconoclasm that late '60s psychedelia had had. Punk was great because it brought back youthful energy, anger, and freshness, but that was 35 years ago! There is nothing new or youthful or fresh about punk now. We ought to be able to recognize now that most of it was crap at the time, even while acknowledging that even the crap was salutary. Being nostalgically attached to punk, and punkish attitudes, now is way worse (at least 3 times worse!), way more deadening and stuck-in-the-past, than the nostalgic attachment to 1967 that the punks so despised in the prog rockers.
That said, for all their musicianship and cleverness, indeed, in large part because of it, Yes totally sucked.