I have three contenders and I can't choose between them, so I'll state them all:
'At Swim Two Birds' by Flann O'Brien.
It's a fabulous (literally) piece of metafiction that intertwines three different stories in a beautiful system of self-exploratory literary conceits. A wonderful tapestry of words and ideas. Great fun for the untangled mind.
'The Island of The Day Before' by Umberto Eco.
One of my favorite books ever, this is an epic philosophical adventure that romps through history, science and religion, powered by Eco's linguistic acuity and masterful storytelling. It is pure literary beauty without fault. I read it in the original Italian and then in English, but he always gets translated well.
'If Upon A Winter's Night A Traveller...' by Italo Calvino.
Another metafiction and a superb example, elegantly written and wittily composed. It's short, compact, atmospheric and humorous, as is all his work.