One thing that is guaranteed to suck me into any fantasy series is a unique system of magic. L. E. Modesitt Jr's Saga of Recluce did this with their Order/Chaos dichotomy, the unusual qualities of Channeling was one of the things I found fascinating about all 13 books (so far!) of Robert Jordan's Wheel of Time series. Both great series and I'd highly recommend a read. Modesitt's Saga is less of a commitment, as most books in the series are standalone novels that cover separate events/characters over the span of a 3000 year timeline. There is the occasional continuation of a story, but that's normally no longer than a second book. We'll Just say Robert Jordan's WoT is quite a bit more of an investiture at 14 books at 500 to 1200 pages each..
I'm here to talk about those fantastic series, I came here to talk about one of my newest/most favorite fantasy series which would be the Mistborn series by Brandon Sanderson. Besides turning the common fantasy trope on it's ear by starting the series 1000 years after a Pre-Destined Hero had failed at saving the world, which has been plunged into complete and utter domination by the Lord Ruler(not a sarcastic name), Brandon Sanderson's form of magic, Allomancy, though limited in some ways, was still complex enough in it's potential that the first book of the following series has already started shifting what I thought I knew about Allomancy. The idea is that Allomancers can eat trace amounts of different kinds of metal and "burn" them to create specific effects and all the effects, as well as the metals they come from are related in different ways
The effects range from affecting the physical self, other people's emotions, physically pushing and pulling on metal objects, as well as seeing into the future or past. I won't go into too much detail, but I'd highly recommend the either in general and especially the Mistborn series in particular.
That said, what is your favorite fantasy series?