@Call.Me.Ishmael, I completely agree about Ludlum. Some of it is a bit dated now, but if you ignore those references, they're still excellent to outstanding.
In addition to Butcher's Dresden, Heinlein's "The Moon is a Harsh Mistress" and "Starship Troopers" are always gonna be high on my list.
If you like alternate history/one way time travel, Eric Flint's 1632 series. Not your typical sequential read, but worth reading the series timeline to get the "right" order. :)
For Space Opera, try Weber's Safehold or Honor Harrington series. Both get pretty wordy and cover a lot of plot territory towards the end, but I enjoyed both of them.
For pure, simple fantasy, try Mercedes Lackey's 500 Kingdoms series. Kind of a sideways take on some classic fairy tales. For instance, the first one is The Fairy Godmother, and tells the story of what might have happened to Cinderella if the prince hadn't taken her away to marry him.
Piers Anthony's Xanth series is a good read.
If you play D&D, Weber's Bazell series (starting with Oath of Swords) is truly excellent. A talented DM's alternate D&D world, and it works well.
Another excellent alternate history/time travel one is Stirling's Nantucket Event trilogy. He also write a longer series about what happened in the world left behind when the Event happened. That one is called "The Change" and starts with "Dies the Fire".
For Star Wars fans, Timothy Zahn's Thrawn trilogy is really probably the best Star Wars story ever. It's no longer canon, but it's still excellent. And I hear he's re-writing it to fit in the current Disney canon.