It looks like there's a clear majority vote for Rendezvous with Rama. I don't actually know anything about it other than it's by Aurthur C. Clarke.
Read it here:
The discussion thread will be poster in two weeks on Tuesday the 18th
kleinbl00 JakobVirgil eightbitsamurai mhr OftenBen plewemt elizabeth blackbootz flagamuffin
...most people spell it with a Z.
I took a gamble and downloaded it as an audiobook. Finished it four days ago. It's every bit as good as I remembered, with sexual politics every bit as dated. I might be willing to transcode it up to an m4a if there's interest.