I've given up reading people's posts on here as very few of you actually know what progressive rock is [edit: with the exception of dmpoole who does]. What prog rock is in it purest forms is NOT Opeth or Muse or QOTSA lol.
Prog is one of my favourite genres of music so what I would say is listen to the following which are definately prog, and if anyone decides otherwise then give them a swift kick to the ovaries.
Quintessential prog:
King Crimson. In the Court of The Crimson King should be a good starting point for your progression into prog. Follow that with In The Wake of Poseidon or Red.
Emerson Lake and Palmer. If you dont understand music all that well I'd save these guys till later once you get to grips with prog.
Hawkwind. Not my personal favourites or even a regular on my playlist, however, they are a good example of progressive rock.
Gentle Giant. I'm not going to rant and rave about how amazing they are, but if its prog you want, then that is exactly what you will get with GG.
Yes. Many will cite Yes first in a list of prog. Some fantastic examples of the genre are to be found, and if you played the Homeworld games you'll eventually hear a familiar song!
Pink Floyd. This is technically prog. However, not all of their music is too progressive but the examples of the genre within their music are to be found in the following: Animals, Meddle, Atom Heart Mother Suite. These albums have consistently progressive themes in them, whereas some of their other albums will contain prog songs but not an overall sentiment of listening to prog.
I have worked through the discographies of ALL the above with the exception of Yes and Hawkwind [working through Yes as we speak]
The following are sometimes classified as prog, due to some elements of the progressive genre appearing in their music. However, they are in some cases more modern than the above and also contain more metallic elements within:
Tool. Fantastic I think, but I see them as metal first and foremost. However it is very easy to put forward a case for them being progressive.
Dream Theater. The misspelling of Theatre always bugged me lol. I like this band and have gone through their discography. I recommend the album A Change of Seasons to see some modern prog at its best; especially the title song. Octavarium itself has some elements, and you can find the rest for yourself.
I really hope that this gets read and doesnt disappear in a day because I've spent months of my life listening to prog and would love more people to get into it rather than panning it as 'gay' or 'boring' because they prefer tri-chordal filth that gets published in many genres and especially in the mainstream genre which is **** to be honest.
I wish you many many happy hours of listening!