Their longest songs are usually the best, but my personal fave is Trois Miniatures, kind of blew me away with the intricacy.
From the Génération Sans Futur album, another one of their best; all of the tracks on that album were great, but my personal favourite is Divertissement. I liked how it was far more erratic compared to the other tracks, switching from a slow tempo to a much faster one with a harsher sound within a short period of time. Trois Miniature is great, but a bit too relaxed for my taste.