It all depends, are you looking for "hard" sci-fi (where the science aspect is more strongly grounded to real science) or would you prefer something that plays fast and loose with reality?
For the latter, I've found David Brin's Uplift series, particularly Startide Rising, to be a good read. It's got dolphins, and they crew a starship

.
Another series I enjoyed was Cherryh's Foreigner series (six books in all: Foreigner, Invader, Inheritor, Precursor, Explorer and Defender. Maybe not in that order though). Essentially about the representative of the human minority on an alien world and his dilemas in dealing between the two goverments. Interesting aliens who are
just different enough to be interesting.
For hard sci-fi, Carl Sagan's Contact is a good read, much better than the movie they made out of it. Arthur C. Clarke's 2001 and 2010 ( I was never able to get very far into the others) are worth reading, as is Rendez-Vous with Rama.
There's the quintessential cyberpunk of William Gibson. I'd recommend his first 'trilogy' (Neuromancer, Count Zero and Mona Lisa Overdrive).
I'll take it for granted that you've read Dune.
Oh, I forgot to mention Spider Robinson. he's not for everyone, fairly sledgehammery with his politics (left wing ex-hippy), but Callahan's Crosstime Saloon and Time Traveller's Strictly Cash are fun, as long as you enjoy truly terrible puns.
Ok, editing again because I forgot Phillip K. Dick, particularly his short stories, his prose isn't all that great but he does have a talent for messing with your head.
(edited to add Spider Robinson to the list, and again for P.K Dick)