DT has never been a lyrics-oriented band. But they have certainly had their share of good lyrics.
For me, it's harder to compare the lyrics of a project like The Astonishing or even Scenes From a Memory, since the purpose of the lyrics on those projects are slightly different: to move along the plot of the overall story. In that vein, the lyrics of both are fine - nothing stellar, but not too much awful, either.
For me personally, the albums that resonate the most are the ones where they seem to concentrate almost as much on the lyric-writing as they do on the music. For that reason, for me (and I'm only talking about what things I look for and what appeals to me personally), their best lyrics appear on Images & Words, Awake, Falling Into Infinity, Six Degrees, A Dramatic Turn of Events, and DT12, with Awake being the champion. The worst (for me) are on When Dream & Day Unite and Black Clouds & Silver Linings, which coincidentally (?) are my two least favorite albums overall.