As a few others have said, it is difficult to pick an absolute favorite. I'm a moody reader at times, so what I do or don't enjoy changes from time to time. I can say, though, that it usually comes down to 1 of 6 series (in no particular order, and all of which have been read multiple times);
Dragon Prince/Dragon Star by Melanie Rawn
The Mythgar series (not really a series but there is an overall story arc to the books) by Denis McKiernan
Chronicles of Thomas Covenant by Stephan Donaldson
The Shanara Series (well the first three books anyhow) by Terry Brooks
The Novels of Tiger and Dell by Jennifer Roberson (currently reading for the 5th or 6th time)
The Cleric Quintet by R.A. Salvatore
I'm sure I could name (honorably mention) more but that would be a rather long list.