I am really into Bousquet's Studies. Have spent about 6 weeks on them. They are not the hardest studies out there. That honor (for me) might go to Saint Jacome's 12 Grand Artistic Studies.
But I believe if you wanted to you could work on them for months and months. They contain everything. Any of numbers of different time signatures, wierd metering, Everything from Key of C to 5 flats or 5 sharps. Accidentals all over the place. Lots of Turns, mordents, and grace notes. Just love them. And the beauty is written in 1851. Just amazing etudes (to me)