The prog concept album is definitely the guiltiest of guilty pleasures and I confess I lost my taste for this kind of thing. I don't really do this type of music anymore, but I'd pick these:
1. Genesis. The Lamb Lies Down On Broadway. Still sounds good 30 years later. Arguably, this should count as a great "last" album.
A few horrific months later, you find Phil Collins singing 10 minute songs about mice. Gated drums are just a few years away.
2. The Wall. Sorry they made the bad concept movie out of this. It's still respectable.
3. Tales from Topographic Oceans. The height of prog rock excess and Jon Anderson flakiness. Insufferably great. I used to actually like Relayer too. Unlistenable.
4. Gentle Giant - Interview. The songs are the interview. Wow. This one was very hard to find, and I liked it best because it had less of that faux-medieval stuff these guys did early in their career. It rocked harder.
5. Quadrophenia - Well, this is a classic. I preferred the songwriting on this to Tommy, on which I always found RD to be unrelentingly annoying. Wait. I always find Roger annoying. I've channelled all that I couldn't understand about the who into a deep passion for the very Who inspired Guided By Voices. GbV has led me back to enjoying the Who a bit.
6. Welcome Back My Friends...no no no, spare us we were just leaving, not arriving....
7. Songs from the Wood. More of that pleasant neo-medieval stuff that gave you zits and made you feel vaguely intellectual. Maybe the Gentle Giant records were better than this.
8. Do you count Muswell Hillbillies or the Village Green Preservation Society? Nah.
I'm sorry. I'm a prog dropout.