metaphysician Peter Simons.
Here's a bit on the earlier generation.
Bolzano studied Kant carefully, disagreed strongly, and set out a reasoned realist alternative informed by his mathematical brilliance as well as his Catholic faith. Brentano made it his mission to rescue philosophy from the disaster of German idealism, taking his initial orientation from Aristotle. Both were persecuted for their perceived offences against church and state orthodoxy. Bolzano had to give up teaching, and celebrating mass or hearing confession, so concentrated on writing large systematic treatises. His Theory of Science completely refashioned and extended logic, but like his other large works on religion, ethics and mathematics, the forced obscurity of its publication in a small town in Bavaria meant he was not properly appreciated for generations, and his work has still to be fully evaluated and absorbed into the tradition. I rank him as the greatest of all nineteenth century philosophers, and as his work comes to be better known, especially after the publication of the English translation of Theory of Science, I expect others will come to share this (currently, extreme minority) view.
Brentano follows after him in my ranking. He continued teaching for fifteen years after losing his professorship in Vienna, and unlike Bolzano, he was unsuccessful at putting treatises together. On the other hand, his lectures and soirées were legendary, and his many inspired and brilliant students went on to become famous in their own right. His writings, the major part still unpublished, are far from easily accessible, and a settled verdict on his work and its importance is not yet at hand. For example, his famed reintroduction of the concept of the intentionality of the mental, which kick-started the phenomenological movement and much else, occurs in a book, his Psychology from an Empirical Standpoint, which he wrote basically in order to get the Vienna job, and of which only the first third appeared.
Meinong is an interesting contrast to both. Practically all of his work is accessible in print, and he had the fortune to become known in the Anglophone world earlier through attracting the attention of Bertrand Russell
All three of these philosophers enjoyed little success in Germany, at that time the leading philosophical nation. Their views were simply too greatly at odds with those of Kant and neo-Kantianism, then the prevailing trend. Even today, it’s hard to find a philosopher in Germany who does not regard Kant as the supreme modern philosopher. By contrast, Edmund Husserl, Austrian by birth but German by choice, did move away from Austrian realism to German transcendental idealism. While this dismayed some of his followers, such as the brilliant Adolf Reinach and the young Roman Ingarden, it meant he fitted in better to the wider German milieu,