These are just a few fantastic series I feel are worth reading
1. The Baroque Cycle - Neal Stephenson
This is a trilogy: Quicksilver, The Confusion, The System of the World
Stephenson has been one of my favorite authors since I read Snow Crash many years ago. The Baroque Cycle is closer in style to Cryptinomicon than Snow Crash, but even that comparison doesn't begin to describe the series. The first book, Quicksilver is slow to get started, but the payoff when you reach the end of the 3rd book in the series is nothing short of amazing. You'll meet famous people in history throughout the book; Isaac Newton, King Louis the 14th, several members of the English Royalty, and almost every great mind of the Enlightenment. And for those of you who appreciate a beautifully-bound hard cover, this series is available in a special edition that looks amazing.
2. The Bas-Lag Books - China Mieville
Perdido Street Station, The Scar, Iron Council
Another trilogy of books, but the stories are not connected. The only thing they have in common is the fact that they all take place in Mieville's vivid and fantastical world, Bas-Lag. The world has everything a monster fan could ask for; giant consciousness-eating moths, deadly trans-dimensional spiders, killer disembodied left-hands, and all the political strife you can handle. His books are a trip to another world. Start with Perdido Street Station, move on to The Scar, and wrap it up with The Iron Council. And if you want even more, you can get one more little taste of Bas-Lag in the short story Jack.
3. The Song of Ice and Fire - George R.R. Martin
A Game of Thrones, A Clash of Kings, A Storm of Swords, A Feast for Crows, A Dance with Dragons
This series is the Tolkien killer. The characters are packed with good and bad aspects. None of them are perfect, and most of them make you want to tear your hair out. Martin isn't afraid to treat his characters with a ruthlesness that can only be found in real life. He kills them off in fantastic ways that you REALLY don't see coming. The only bad part of this whole series is...it's not done yet.
Here is a series that MAY be worth checking out, if the follow up books tie up some loose ends.
4. The Prince of Nothing - R. Scott Baaker
The Darkness that Comes Before, The Warrior-Prophet, The Thousandfold Thought
This series starts out incredibly strong. Baaker's world building is on-par with China Mieville; the world is fully realized with a history that is as mysterious as it is vibrant. The title character, The Prince of Nothing, is absolutely fascinating. He is part of a monastic group of people who have lived in isolation for centuries using breeding practices to create an individual with inhuman speed and perception. My only complaint about this trilogy is the ending. The Author left FAR too many loose ends dangling. The next book, which is not part of the original trilogy, has just been released and while I haven't read it yet, it seems to wrap up many of the plot strands that I felt were left dangling. This series is unique in tone and style and worth checking out, I am looking forward to reading the follow-up books to see the conclusion of the story.
That's all I have for now, I'd love to hear your thoughts!