Well it depends what you mean by 'best' - top 10? Top 20? One thing I love about the band is that they always seem to be able to pull out at least one absolute classic song per album, even this far into their career. My top 10 happens to include one song from every album between Train of Thought and DT12. So since I became a fan, each album always had at least one standout, which is pretty unique among the bands I listen to that have lots of albums.
That run was always going to end at some point, and it happened with The Astonishing. However, I still really enjoy the album and think there are a lot of good songs and great ideas on it. Since DT has so much incredible music, I could easily define top 25 or even top 30 as being 'among their best' - in which case a few Astonishing songs would qualify.
A New Beginning - my favourite from the album and it would probably just about squeeze into the top 20
The Gift of Music - I tend to prefer the longer songs, but I love this one - easily my favourite 'short' song that they've ever done
Three Days - Another one of my favourite short DT songs, really cool little track
Moment of Betrayal - Not much to say about this one, just a good, solid song
The Path That Divides - One of the best tracks on the album, it might sneak into my top 25. It also kicks off probably my favourite 3-track run on the album
My Last Farewell - Not sure if I'm including this on not just the strength of itself, but also how it fits together with the 2 preceding tracks, but I do really like it (I was going to include The Walking Shadow too, but it maybe doesn't quite make it as a standalone song)