I would say it's the most unique of their albums with all of the features and really has an emotional/moody vibe to the entire album. It's always hard to say though because Plastic Beach is also amazing, as well as their entire library of music.
Also, try listening to the album in backwards order. It usually gives an album a totally different feel, not always. But Demon Days especially.
I wanna say Demon Days, they had prefected their sound in that album but seltitled broke so much **bleep**ing ground it's crazy, to think the impact that album has had on modern music, so it's a hard question lol I'm not a fan of humanz, I have it and it just doesn't like them and it has too many ft. in it, plastic beach is good but for me personally the later the album the less it feels like them. I'm gonna say Demon Days after all that just coz I think they perfected the Gorillaz style there lol