I haven't read a ton of Fantasy series, being a relatively new fan of the genre, but my favourite series so far is The First Law trilogy by Joe Abercrombie. He's a terrific writer, and the series has a great cast of characters. Glokta, in particular, is one of the best characters I've come across in any series.
Other series I've read include:
The Wheel of Time by Robert Jordan - Good series, but far too long and has way too much filler
The Dark Tower by Stephen King - Books 5 & 6 were boring, but otherwise a good series
Harry Potter by J.K. Rowling - Not exactly an 'adult' series, and fairly predictable, but fun and interesting
Mistborn trilogy by Brandon Sanderson - I just finished #2. A pretty good series, so far
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