Aspa's stuff is all excellent. Well, except for the last one, which I've never even heard of, so can't vouch for.
Basically any sci-fi made in the past decade or so, or which "includes fantasy aspects", is pretty much a self-indulgent fantasy wank or some kind of cash-in on a popular universe (i.e. Dune, Star Wars, video games), so I'd stay away from those. Anything made before, say, the thirties isn't generally going to be science fiction so much as fifty years ago plus some aliens who're basically the same as us only eviller. Anything that falls outside of those parameters is generally passable.
In particular, Bester is the most incredibilitastic read ever, Asimov, Clark, and Heinlein are good if you want lots of metaphor and subtext (which is really what sci-fi's all about), and Pohl, Lem, Vonnegut's wierdnesses and those hordes of Russian authors are all pretty decent reads. Um, provided you can either understand Russian or get it translated.
And short stories from the old 60s sci-fi rags, if you can find any. They're generally pretty good.