I always liked the mythic and literary allusions in Zelazny's work, and he was an author who could write convincing characters. (Strong characterization and natural-sounding dialogue hold more weight than clever logic problems and puzzle box plots for me, though I know that's a matter of personal taste. Some people enjoy sf, particularly classic sf, for exactly those qualities, and that's perfectly valid.)
Niven's Known Space stuff
is space opera, without a doubt. But it's
good space opera. His characters are believable, he can write natural-sounding dialogue, and he's good with big picture concepts. It doesn't hurt that the man knows science (and isn't afraid to use it).
I liked OSC's stuff, but his homophobic leanings have really turned me off of him as a writer and a human being. : / The only Lem I've read was Tales of Pirx the Pilot, back when I was in my teens (which I enjoyed). As for David Brin, I must have everything he's ever written sitting on my bookshelf. : )
As far as other sf authors I like, there are probably too many for me to list here. >.> I'm a big fan of Alastair Reynolds. C.J. Cherryh's Foreigner series is a favorite of mine. So are Dan Simmons' Hyperion Cantos, particularly the first two books. (There are times when I think he'd've been done better to just end the series with The Fall of Hyperion.) Everything William Gibson does is golden, whether we're talking about his cyberpunk or post-cyberpunk oeuvre.
Sorry for the derail, folks. Guess I'm just happy to discover that there
is a sf-GAF.