I would have to say the entire lensmen series, by E.E. Doc Smith, for science fiction, as I've read it several times (I don't tend to re-read most series most, no matter how good), although a few fantasy series have been pretty good as well.
The Deed of Panksenarrion series, by Elizabeth Moon, was great too, showing a degree of realism for it military aspect that most fantasy never gets close to, yet still has a great fantasy story, in a world that just welcomes you.
The Darwath trilogy, by Barbara Hambly, was pretty good too.
Currently reading a good, in depth series, the malazian book of the fallen, starts with Gardens of the Moon, by Steven Erickson.
All of the above, I recommend for those who read the genres.