I consider these books to be the best works of Bruce Sterling:
Schismatrix Plus is a collection containing the novel Schismatrix as well as Sterling's Shaper/Mechanist stories. I have read many of Sterling's other stories and can recommend any of the collections without reservation, but this collection is a good introduction.
Heavy Weather is a slightly fantastic, but mostly "hard" science fiction novel about the future of global warming and what the resulting storms might look like.
Holy Fire has as its main character a wealthy elderly woman who undergoes physical rejuvenation, and in the process becomes a new person.
Zeitgeist is something quite different for Sterling: more magical realism than science fiction, about a petty-criminal girl-band producer in postmodern Eastern Europe, and I liked it a lot more than I thought I would, just because it goes much further than I imagined it would.
The Difference Engine is a collaboration with William Gibson, a steampunk "future history" about a world in which Babbage's designs enable the computer revolution early.
And one final pick:
Involution Ocean was Sterling's first novel, originally published as part of the Harlan Ellison Discovery series. It is not a great novel, but it is an intriguing takeoff on Moby Dick, with a deadpan narrator, and written in a style a bit like China Mieville. I have read it several times. Don't read it as an introduction to Sterling, but it is short and memorable and deserves to be back in print, while The Artificial Kid, which has been reprinted, is not as interesting.
I have not yet read The Zenith Angle, but it looks promising.
Sterling and William Gibson are often lumped together. I admire Gibson and first read his stories in Omni magazine when I was very young; these stories are collected in Burning Chrome, which is an excellent collection. Gibson is a beautiful prose stylist; I admire his short stories greatly, and Neuromancer is rightly considered a classic, but his recent novels seem to have a sameness to them. In Pattern Recognition Gibson seems to have actually thrown in the towel and stopped attempting to write about the future, writing instead about the present, his point apparently that we are now living in a science fiction story. This seems to me to be Gibson admitting that he is getting old and his foresight has gone blind. Sterling seems to be the real idea man, and while perhaps not the lovely stylist that Gibson is, he is still a very versatile and assured writer, with a touch of manic comedy and a constant sense of the surreal.