Ooh boy! Prog is one of my favourite genre, too. Just start with Polygondwanaland then, it’s their most proggy album.
They have so many varied albums, I will list a few which I like. You can also use this website to guide you through and explore their discography, www.get-into-gizz.com
I also really recommend Nonagon Infinity, and Ice, Death, Planets, Lungs, Mushrooms and Lava for some amazing psychedelic/rock. And some of the microtonal albums (Flying Microtonal Banana). A lot of people are not very much fans of the slower/more chill albums, but Butterfly 3000 is one of my favourite albums from them. Omnium Gatherum’s first three tracks are worth listening, too
Apart from these, Infest the Rats Nest is an absolute banger of a thrash metal album.