I'm going with Farthing, as I read Among Others recently and thought it was excellent. Earth I've read, don't care to reread it, and Red Mars I've been halfway through for many years. Rama I read way back when it was first published, while good, it's not calling to me. The others I can't quite tell, maybe the discussion will bring out more compelling reasons for me to switch my vote.
based in some strong recommendations from the GR peeps, there's a copy of 'ammonite' sitting here waiting to be read. but ditto, just read the marvelous 'among others', so 'farthing' would be alrighty too.
So given that I had to vote for Farthing