Souvenirs is the best, but I should warn you, it's not very "metal." A good description would be Justin Broadrick from Jesu playing Red House Painters inspired music with French lyrics. It's very muchso shoegaze and very littleso metal.
There's not too much from Alcest out there. Tristesse Hivernale, the demo, is almost unlistenable in terms of recording quality. It makes Towards the Sinister (MDB) or All Faith is Lost (Anathema) sound like high quality studio recordings. Le Secret, Souvenirs d'un Autre Monde, Ecailles de Lune, and Le Voyages de L'ame are pretty much it. The split they had with Les Discrets has one non-album song Circe Poisoning the Sea but it's a mellow acoustic number. Still worth listening to.
If you do like Alcest, Neige has some much more "metal" releases such as Vastiia Tenebrd Mortifera from the band Mortifera (Neige + that fat piece of shit Noctu), the Ameseours releases (Ruines Humaines and the self titled full length) which mix post punk, shoegaze, and black metal, and the Lantlos releases Agape and .neon which are post black metal, Agape has some sludge too.
Here's a good example of a more metal song from Neige: