I usually answer Pink Floyd for this, but I do like some of their stuff a good bit. Kansas is another option, though I do like some songs. Steven Wilson is one I don’t like near as much as everyone else, though I respect him as a musician and producer, and I like some Porcupine Tree. I haven’t heard that much Jethro Tull or ELP, but what I have heard has not compelled me to dig much deeper.
I’ll go with Jethro Tull and ELP.