Ugh Idle why would you do this to me? I have a compulsion to do these top ten list things but trying to judge my favorite book series against on another... I'm gonna make it easier on myself by listing series as one item and not over thinking it with long explanations.

1. A Song of Ice and Fire series by George R.R. Martin - Duh.
2. Malazan Book of the Fallen series by Steven Erikson - Tons of characters with a story that seems to span only a few years but has important shit reaching back hundreds and thousands of years. One of the most intricate and far-reaching plots I've ever seen in a series of books. First book was kinda lackluster, but it picks up real quick after that.
3. Wheel of Time series by Robert Jordan and Brandon Sanderson (who completed it after Jordan's death) - Starts of as a kind of typical "random farmboy is actually the chosen one" thing, but it gets a lot bigger and better as it goes on.
4. Good Omens by Neil Gaiman and Terry Pratchett - Fuck it's hard to weigh single books against series, but it's damned good. I've got a generally higher opinion of the Wheel of Time though, so it gets the #4 slot.
5. The Stand by Stephen King - Notable for being the least fantasy type book on my whole list. It's a huge book full of great characters who develop in interesting ways, and it might've beat out Good Omens if not for the ending being kinda underwhelming in typical Stephen King fashion.
6. American Gods by Neil Gaiman - The most recent addition to my top ten. The basic premise of it is a very interesting take on gods and religion and American culture, where all the various gods of the past actually exist as people and the things modern people worship (like TV and the internet and such) also have humanized forms. It's neat.
7. Mistborn trilogy by Brandon Sanderson - Probably the coolest magic system I've ever seen. Instead of magic just being a deus ex machina tool, in Mistborn it's more like a science with real material costs and such. It's also a cool story with good characters and blah blah blah.
8. Coldfire trilogy by C.S. Friedman - This is the closest thing to sci-fi as exists on my list, but in the end it's really still a fantasy thing. It's set on some other planet way in the future, but people have lost all their tech shit and there's magic going on and such.
9. The Black Company series by Glen Cook - Like my #2 pick, this one was kinda hard to get into. It's the story of a mercenary company, called The Black Company, and it's told through the company historian dude's record keeping. It's by no means a standard fantasy thing, lots of weird shit going on, but I got into it and ended up liking it a lot.
10. Discworld series by Terry Pratchett - Fantasy-leaning comedy is best comedy. Tons of goofy hilarious crap in these books, but also some surprisingly great characters.

Honorable Mentions
Lord of the Rings series by J.R.R. Tolkien - Can't be a fantasy fan and not give props to LotR. Doesn't make my top ten because it really does drag ass through a lot of the books, kinda crap pacing, but it does still manage to be an interesting read anyway and without it fantasy as a genre as it exists today probably wouldn't be a thing.
Redwall series by Brian Jacques - My first foray into fantasy, courtesy of my grandmother giving me the first book (called Redwall, like the series itself) for one of my birthdays. Without this book and series being part of my young life, I feel like I would have been a very different person, so I couldn't not mention it.
Shannara series by Terry Brooks - This was the first epic fantasy (by the definition I like, not the one lots of people use that's interchangeable with 'high fantasy') series I read, and it led me to a lot of other great series through recommendations from various sources. Similar story with Redwall here, of not being high quality enough for my top ten but too influential on my younger self's reading habits to not mention.
Sword of Truth series by Terry Goodkind - Used to be my favorite book series of all time, before I ever read any of the ones currently sitting on my list. It felt like something new and amazing and different from everything else when I was 14 and read the first book, but it really deserves an honorable mention for the same reason as the above things for influence and leading me to other stuff. I was first recommended my current top three book series on some forum where I was discussing the Sword of Truth series with other people and asked people what other fantasy series out there I might like since loved this one, so it gets props for leading me to true greatness.