SONGS is fine, but I would say it's one of their weaker albums. That's no shame though, considering the band has put out some really good albums. Personally I consider Second Life Syndrome and Anno Domini to be their best, with Rapid Eye Movement a close third. Out of Myself is really good too.
I'd probably agree with you, ADHD is my favorite (and maybe the best), and SLS "is" the best, but not my favorite. I haven't been listening to Riverside, or prog rock/metal, for a long time, so maybe the fresh ears are adding something to it, but I recall enjoying S.O.N.G.S. when it came out, because of how much they've moved away from their older sound (first 3 albums and EP). So far, ADHD is a more unique album in the band's catalog, and maybe why it's my favorite. However, they have a warmer sound now, not sure how to describe it, it's been developing since the Memories In My Head EP. Less reliance on riffs, more pretty chords, jazz influence, which is where my general tastes have been going anyway.