Am I the first person to suggest
Tad Williams - Memory, Sorrow and Thorn trilogy
Tad Williams - The Shadowmarch trilogy ( Book 3 still not published).
I have a caveat tho' - both the above are not for people who want pages filled with action. It's excellent writing which builds the mood. eg. In the Memory, Sorrow and Thorn trilogy - the first 60-70 pages as you follow Simon around the castle, you get only a glimpse of the momentous events happening around you but you know they are important and wish it was mentioned more "head-on" rather than obliquely but it helps build the atmosphere. A long really well-told and well-realized (in terms of creating a fantasy world) saga which goes against a few Fantasy tropes. In Tad's worlds, the "Elf-equivalents" are always ancient, much more cryptic than in other fantasies, and the buildup to their magics is remarkably dense and pleasing.
I am definitely glad that The Bartimaeus Trilogy has already been mentioned on Page 1 of this thread. I loved it. It's more of a young adult but with a surprisingly mature and gentle way of treating the whole thing.
Mordant's Need is also well-written and while I admire it, I don't *like* it (in the way that I want to "like" a fantasy series). Same goes for Philip Pullman, I think.
And a shoutout to JKR's GOF - I think it has the best 100-150 pages of constant buildup that I've seen in a recent work. (Tho' I really didn't like her writing for #6 and #7).