Guys, why isn't Robert Jordan's The Wheel of Time series been listed yet? Speaking of things I need to reread... I need to reread pretty much the whole series, then I can finally read the LAST ONE. There are fifteen books in that series, each at least 500 pages. x.x
I can't remember some of the actual series names at this point, but there are some more that I think are worthy of being in this list...:
The Discworld series, by Terry Pratchett. Hilarious, intelligent, and profound, and only get better later in the series. Though it's more like a collection of books than a series.
Saga of Recluce, Corean Chronicles, The Imager Portfolio; all by L. E. Modessit, Jr. You want realistic battled and wars? He's got 'em. It all checks out, in terms of logistics, economics, politics...some brilliant scheming here. The author's a former soldier; he knows his stuff.
The Mistborn Trilogy, by Brandon Sanderson. This is the guy that was chosen to finish the Wheel of Time series upon the death of Robert Jordan, and he did not disappoint. He tends to have very cool, original magic systems, and the world he built in his book, The Way of Kings (the first in his own giant series) really impressed me by having the the flora and fauna make sense evolutionarily based on the weather and climate of the planet.
Author: Robin McKinley. Truly a master of the mythical. Her books feel like fairy tales, almost. And they don't explain everything to you...some things you have to figure out yourself. One of my very very most favorites.
There are a lot of others, but these are the big ones. I may or may not stop by later and add more to this list.