The entire Night's Dawn trilogy by Peter F. Hamilton(trust me, the premise isn't much, but it's good space opera with Al Capone, the good old fashioned supertech and a little metaphysics thrown in for good measure, the whole thing is about 3500 pages, or 6 paper backs(USA) or 3 hardbacks(UK, country of original publication))
Anything by Clarke, as Maeg said.
Most anything by Asimov, I liked the Robots and Murder series(The Caves of Steel, The Naked Sun, and The Robots of Dawn) which is set literally days after Daneel Olivaw's inital construction, I understand he plays a significant role in some of the more recent Foundation books.
The Icarus Hunt by Timothy Zahn(not a SW book)
Ben Bova if you're want some hard science in your space opera
Most of my library comes from the Science Fiction Book Club so I apologize if some of the Titles I mentioned are unavailable.
Oh, I almost forgot to mention:
The Voyage of the Space Beagle by A.E. van Vogt
This book is more a collection of Short Stories by the Author, but every single major appliance in SF today can be traced back to this book, most especially the concept of the Alien as seen in the popular films.
Also, though I've only read one, the Lensmen series.
