Yeah, hard to do on that score. Most people I know rate "Dark Side" more highly than the Wall but fuckit, they're both brilliant, groundbreaking albums.
The Wall is so fluid and cohesive. It's not just lyrically and poetically a story; it's a story musically. The musical motifs and the musical changes that set the mood really have just as much to do with the album being a story as the lyrics and poetry. To me, it's truly the most artistically valuable album the music medium has. The other two are just kick ass.