Hmmm...Tough choice.
That said, I think that, at least until someone else can convince me otherwise, A Farce to Be Reckoned With has to be up near the top of the list...if for no other reason than that it's the last book Zelazny got to contribute everything he intended to...
Others on the short list: just about anything by Neil Gaiman (though it seems like American Gods may be his best at this point), The Long, Dark Tea-time of the Soul by Douglas Adams (like everyone else, I wish he were still with us, but to write more Dirk Gently, more than continuing Hitchhiker's), the Islander series by S.M. Stirling (which I count more as one book than three) and just about any of Harry Turtledove's non-Detina or Derlavian War books...(Harry Turtledove does magic...ummm...not so much?) Honestly, there's just too much in the genre to make one choice...I haven't even touched on Saberhagen or Piers Anthony (and I can't believe none of y'all called me on that goof while it was up!!) yet...