If you're looking for eloquently-written SF, I'd have to recommend the work of Jack Vance. These days he's mostly remembered for "The Dying Earth", which is excellent but is essentially fantasy; most of his work, though, is SF, from planetary romance (Big Planet or Planet of Adventure) to space opera (Demon Princes). The latter, which was originally published in the 1970s, is probably the pinnacle of his SF writing.

Vance's biggest strength is his eloquent, very urbane style, especially in dialogue. He's probably the best writer I've read at coming up with neologisms that sound like real language; not really "can look up in the dictionary", though.