Down is the best for me. Sanguine Sky is a close second place, and Lotus is in third.
Though, none of those three are that far ahead of the other tracks - because all the tracks are good. Illumination has gone down for me as being one of the small handful of albums made in the last ten years or so where every single track is good. So many albums made in the last few years have a couple of good songs (for singles) then just boring filler for the rest of the album, but with Illumination (and in fact, all of Tristania's albums so far), every single track is good. In a way, it seems a bit unfair to me to pick a favourite track or name certain tracks as being better than others, because really, they all deserve praise.