I was avoiding sci-fi/fantasy cross genre titles as that could be arguable by some. But as others have said it I will throw in a few of my favorites.
I definitely love McCaffrey's Dragonriders saga, although I feel her continuation began to get a bit drawn out. I felt she had gotten it too drawn out and was losing touch with the stories. Then I read Masterharper and she brought it all back home for me. Either that or the fact that a book focused on my favorite character finally was written and managed to tug on my emotions with how it played out just hit me right.
And as already mentioned, Heinlein has some very good cross-genre stuff. And while I agree that early on he seemed to be drifting toward a very socialist bias he seems to have softened in his later years as it became more open minded. I know the message kind of went from a socialist point of view to a "who knows and does it matter" type attitude.
I think the sci-fi meets fantasy thing is definitely something that can't be ignored as there are numerous tales of knights and wizards fighting dragons, oh but we have hovercars and lasers too.
Currently I am listening to the second book of a podcast novel by Mercedes Lackey called "The Secret World Chronicles" that tells of an alternate universe where metahumans (superheroes) arose during World War II and because of it the modern day world has a few vast differences. For instance Tesla's broadcast power became a reality and power suits, created with nano-fiber, for weaker metahumans were built. But at the point I am at there has become a difference between true metahumans and those who have magical powers. Suddenly fantasy has drifted into the story and part of the problem arises when those with magical powers cannot always control it and sometimes lose control of even themselves. And then there is now the debate of whether or not a magician with natural abilities qualifies as a metahuman superhero.
It is definitely a unique twist of the sci-fi/fantasy crossover, especially the way it has crept into the story mid-way through the second book.
And that reminds me: do podcast novels, or "podiobooks," count in this discussion? I can think of a few that rank up there in quality. Also others that rank quite high in entertainment value.