I'm going to pretend this isn't easy by being conversational for a bit.
Obviously the worst band of all time is a band you've never heard of. It's three or four little shits in a basement or a garage, not knowing how to play their instruments and doing it all for the wrong reasons. They're co-opting culture and creating nothing, and they sound a little like a sheep with chronic neck pain.
So, as I am often forced to do on blog, I will interpret this question for the masses. In this case, you want me to say the name of a band you've heard of. If I say that the band next door to me is the worst ever, and I tell you their name is 'Donkey Dick' you'll be all like, 'Who's that?' or 'Nice name.'
The worst band of all time is Blink 182. Now if you're a fan, feel free to take a moment to be terribly offended before realizing that you're a huge piece of crap.
Lots of bands have had 'catchy' and nothing else, but these guys really take the shit-cake. Is that supposed to be punk music? There are some motherfuckers who roll in their graves when they hear the term 'pop-punk', so don't pull that oxymoronic ace from your badly designed tattoo-sleeve in response. This is music meant to sell music, with no soul, content or impact.
Don't get me wrong. Some things are for fun. If you listen to this band - or any - and you have fun while doing it, more power to you. Spread that love. But by all the criteria that one such as I must use to assess the merit of a thing - based on questions that include the assumption of critical analysis to justify concepts as lofty as 'good' and/or 'bad' - then I must hold to certain standards and say that a conversation applauding the value of Blink 182 is the social equivalent of a wet fart in a formal gown. That's, to be clear, a terribly off-putting noise followed by the spread of fecal stains on an expensive fabric.
Somehow, their music manages to co-opt popularity itself. It's a flavor, an ad, a concept. It is not melody, rhythm and soul. It is vacuous and it sounds about as challenging, important or subversive as the speech given by the guy representing the Academy at the Academy Awards.
Short Answer: Again, like what you want. I never make fun of people for liking something. But if you're rolling over into the land of quality, of bests and worsts, then you have to be able to back up your argument. That's why Blink 182 is the worst band ever. How the fuck do you explain that they're good? And if you're asinine enough to try, just know that somewhere, every member of Rush is politely trying not to laugh at you.