I can't say that I count any of those bands as favourites. In fact I don't like Dream Theatre and well, I won't get into Opeth.
However, prog-rock/metal is good stuff. Try some
Porcupine Tree since you like Pink Floyd. They've gone through different stages though. Their last few albums have been more prog-metal, but not as heavy as like, Opeth, and some of their early albums were just odd... and some of their albums were psychedelic. I recommend the albums, Stupid Dream and In Absentia myself. Luckily their myspace has their greatest song on it, Trains:
MySpace.com - Porcupine Tree - London, UK - Progressive / Ambient / Experimental - www.myspace.com/porcupinetree
Another good prog-metal band, and I use the term loosely because they incorporate a lot of different genres, is
maudlin of the Well. I love them. Their album, 'Bath' is one of my favourites ever.
There's also The Mars Volta but I'm sure you've heard them.
The
Between the Buried and Me album, Colors, is some nifty prog-metal, as well.
Well, prog-rock/metal are pretty broad terms so I could go on for a while so I guess this is good!