imho nothing comes close to Clarke, and the space opera as sub-genre is quite varied and diverse ranging from Doc Smith's Skylark and Lensman series to Peter F. Hamilton.
But I've found a couple of interesting books in terms of the space opera/adventure theme although they do not as such contain lots of gratuitous space battles.
Stephen Baxter's Xeelee sequence
Alastair Reynold's Revelation space series
Greg Bear's Eon series
Gregory Benford's Galactic Center Saga
John Scalzi's The Human Division, and there's the Old Man's War series which I haven't read yet.
Although perhaps not considered Space Opera; Kim Stanley Robinson I find quite enjoyable too.
I'm afraid Peter F. Hamilton's Night's Dawn Trilogy was not for me.
Iain M. Banks' Consider Phlebas also did nothing for me. Shame really, because there is a whole Culture series available.
Check out
http://en.wikipedia.org/wiki/Space_opera for more inspiration.