>>510049690
Depends on what areas you're interested in. For History Thucydides for sure, Xenephon's Anabasis (Persian Expedition) if you liked Thucydides then Xenephon carries on where he left off in the Hellenica.
For philosophy read through all the pre-Socratics to start with, it's very interesting and doesn't take a considerable period of time. For mathematics I haven't read anything cover to cover, just swaths here and there but it's just what interests you. There's certainly some great stuff though, especially if geometry interests you.
I Hippocrates is kind of interesting to read bits and pieces of as well. Arrian's Anabasis on Alexander's conquest is solid. Any areas in particular you're interested in?