I will arrive to Budapest Airport around 23.30 and wonder how to get to the city center? When does the last public transportation bus leave from the airport?
Thank you for the question! The last bus (200E) that leaves from the airport to Köbánya Kispest (where you can change to the metro) leaves from Budapest Airport at 22.52 every night. That is quite early, but this is timed in such a way that you will get to Köbánya Kispest just in time to reach the last metro leaving from there at 23.15 (if the bus is a bit delayed you will though not reach the last metro).
In your case this means that you will not reach the last bus and be able to use public transportation with such a late arrival. You could therefore consider a private airport transfer, or the usage of the official Airport Taxi or a minibus shuttle at the airport. The usage of the minibus shuttle is probably the cheapest, but then you will not be able to leave the airport at once, but you will have to wait for a while for a group to come together, and only then you will leave the airport, which is not the best, at least not when you are tired and want to get to your hotel in a hurry!
Enjoy your stay!