Ashes is definitely their best. Marrow Of The Spirit is my second favorite. That album took a while to grow on me, but damn, the atmosphere on that record is unreal. Even the murkier production, which I disliked at first, ended up being a positive for me because it really enhances the atmosphere. Plus you've got songs like Into The Painted Grey, Black Lake Nidstang, and Ghosts of the Midwinter Fires which are all fantastic.
The Mantle is behind those two. The Shadow Of Our Pale Companion might be their best song (though I might instead give the nod to the first two parts of Our Fortress Is Burning) but as a whole I don't think it's as strong as the above two records. Then you've got Pale Folklore, which is a bit rough around the edges but still quite good, and finally the Serpent And The Sphere, which is unfortunately their weakest. Really would have liked them to go out on a higher note.