For me it's Realm of the Elderlings by Robin Hobb, each series within it stands on its own and is then beautifully brought together.

I feel WoT loses it's way in the middle, there are some entire books with minimal plot advancement. This may be hersey but I also found book 4 of LotR, Frodo and Sam walking so tedious. (Despite those criticisms I do love them both).

ASOIAF is up there right now but feel you can't judge until a work is complete.