His Riverworld series was one that I read several times since my father introduced the series to me in the early/mid 80's. I also read his Dayworld Trilogy. I didn't really explore the rest of his work but Riverworld was a very enjoyable series (it also got turned into a roleplaying setting with the release of GURPS Riverworld in 1989) and the one that stands out for me.
Farmer had a good run though, he was 91. Might dig out Riverworld again for another read to honour his passing.
RIP: Philip José Farmer.