I think I'm the first to say this in this thread, so maybe not so well-regarded, but I once gave Devin Townsend a chance (with three DTP albums I bought used at a local FYE many many years ago), and I just could NOT get into it. The three albums I got were Ki, Deconstruction, and Ghost.
I've since not given him a chance at all as my first experiences with his music were pretty grating, IIRC, so I've just not gone back, despite how much I see/read about him and his music, and his work seems to be fairly popular, at least among some prog/metal/prog-metal circles.
Another I've tried getting into and just could not get is Jethro Tull. Apart from TAAB and Aqualung, I've never really gotten into them. I tried really hard when I was first discovering prog over 15 years ago and dove into all the classic 70s prog bands and albums. Bands like Pink Floyd, Genesis, Yes, Kansas, and King Crimson all clicked with me right away, but Jethro Tull just never really clicked with me outside of those two albums. I tried, and even own several of their earlier albums, but nothing just really worked for me. Same goes for other classic 70s prog bands like Van Der Graaf Generator and Gentle Giant. You'd think my love for Spock's Beard might have helped me get into GG, but it just never happened, and with VdGG, I guess I just found some of the music and Peter Hammill's vocals to be a bit much to listen to at times.
One more I've tried, and kind of enjoy but just never really latched on to is Opeth, despite owning almost all of their albums (during a phase of my prog discovery where I bought a lot of albums by a lot of the more well-known, well-regarded bands). I recognize their talent and all, but maybe their music just never lined up with my tastes.
-Marc.