Symphony Masses is almost as good as Beyond Sanctorum. It's perhaps the weirdest album they've released, and can be a bit hit and miss due to the insane experimentation, but when it works it's every bit as impressive as Beyond Sanctorum. Apart from those two albums, my favourite is Sirius B, songs 5-11 are the best Therion have ever made in this style imo. The way the drummer uses his cymbals on this album is really something, I haven't heard anything quite like it. It's not the one reason why this album is so good or anything, but I've never heard it mentioned before.