Not their last album before Roger Waters left the band (that was The Final Cut, the album which followed), but it was far superior, and arguably their best album-- and inarguably their magnum opus.
The David Gilmour-led era of Pink Floyd was ok, but it would never reach the fevered heights and sick intensity of the Roger Waters days.
Some of the tracks are based on his childhood, and seeing how many The Wall tours he did. In 2016 he turned it into an opera. So the album is very personal to him.
A lot of the tracks have to do with what both Waters and Gilmour went through as children, as they both lost their fathers to World War II. David Gilmour got writing credit on a bunch of the tracks as well. And given the amount of work that both Waters and Gilmore put into the album, it’s not really right to say that it was a solo project. Not even to mention what Nick Mason put into it. If you wanna cut out Richard Wright’s contributions, considering that he got fired during this album’s production, that would be fair. 
The Wall is absolutely Rogers album. The concept and the songs are all his. Gilmour only received credits on three of the songs.
You can't point to a few guitar solos and then give Gilmour half the credit, it was a great contribution, but even Gilmour would admit that Roger wrote the wall.
More popular, more commercially successful, and more accessible to casual fans. Agreed.
But for magnum opus, I gotta agree with the wall for a few reasons
They made a movie out of it
The ode to the intense para social relationships that revolve around stardom and how a truly crazy creative can take advantage of it in scary ways was not only true back then, but predictive of how much worse it would get in current time.
DSotM always seemed like a lot of good ideas in an unordered list. I felt like they could be scrambled and the album would be similar, except for the first and last songs.. Meanwhile the wall tells a story of pain, alienation, search for meaning, lashing out, and then a quest for self-forgiveness.