I'm bumping Fates Warning. They are considered to be part of the holy trinity of prog-metal alongside Dream Theater and Queensryche, and are probably second only to DT as far as staying the course as a torchbearer group of the genre. A must listen for any prog-metal fan.
Queensryche from 83-94 is also a must. Anything after 94 is optional.
Also, Savatage fills an odd niche in the world of prog-metal While most of their material from the 80's is pretty standard, albeit very good, old-school power metal, from 1989's Gutter Ballet album onward, they took a rather unique artistic path, blending 80's power metal with progressive, neo-classical, symphonic and most notably Broadway influences. A truly amazing band, although definitely very left of field, even in a progressive genre. I would highly recommend them, as well as the solo efforts of their singer/keyboardist/main songwriter, Jon Oliva.
And finally, any of the projects by Arjen Anthonny Lucassen (Ayreon, Star One, Guilt Machine, Ambeon, Stream of Passion). Pretty diverse, but always with heavy doses of old school 70's prog-rock. If you're into the retro sound, I would recommend them as well.