There's a very nice piece on The Guardian by John Self called Famous for the wrong book. It points out that many authors are famous for a book that is not necessarily their best. A key example being, "why has a title like Louis de Bernières' Captain Corelli's Mandolin risen to the top, overshadowing his much better earlier novels such as Senor Vivo & The Coca Lord ?" Something with which I would agree entirely.
I am tempted to get controversial right away by suggesting that Joyce's short story collection Dubliners is far better than anything else for which he is better known, but that might be my anti-modernist prejudices so let's not go there. Mrs Gaskell however is so very much more than Cranford - Wives and Daughters is my favourite though as she did not finish that maybe it does not count. North and South is superb too.
At the other end of the literary scale, fond though I am of Stephenie Meyer's sparkly vampires, The Host is a better book than any of the Twilight series. It is psychologically fascinating and deals with the problems caused by a society being too perfect, not Stepford Wives exactly, but lacking the anger and selfishness that makes the human world interesting. Where would fiction be without conflict? The Host would make a great holiday read.
The premise works with poetry too. Whilst Sylvia Plath's "Daddy" is great for a dramatic reading to a class of 15 year olds it is by no means her best work. I much prefer "Morning Song" (the one that begins "Love set you going like a fat gold watch".) Similarly Larkin's much quoted This Be the Verse is far from his best, though as I have a reputation to maintain as Larkin's most ardent disliker I daren't offer an opinion on what is his best. If anyone would like to recommend something really, really, really good by Larkin you could ruin my 20+ years loath-affair with his work, a loathing which has given me enormous pleasure. Disliking Joyce's best known works, by comparison, is quite boring!
It occurs to me that, as many of us are put off an author after reading the best known work, perhaps we could share some lesser known highlights from well known authors. For me Anita Brookner fell dismally with Hotel du Lac. Is this really her best or can you recommend something else? Similarly I've twice read The Ghost Road by Pat Barker trying to get the point. Is there a better Barker than her Regeneration triliogy? Any thoughts there? And which writers do you think suffer for being famous for the wrong book?