I've got 14 series (15 if you count 2 from W.E.B. Griffin) that are done (a couple might come back). So that you don't have too much to read at one time, I'm breaking this category up into 2 posts. Here are the 1st 7.
J.K. Rowling and Harry Potter. I loved all 7 of these books (I didn't like #4 quite as much as the other 6 because the 1st part of the book takes place at the quidditch tournament, and I liked them at Hogwarts the best - aren't you glad you asked?).
Larry McMurtry wrote 4 books with Lonesome Dove being 1st (1985). Did you know that there were 3 others? After LD, he did 2 prequels, Dead Man's Walk (1995) and Comanche Moon (1997), and a sequel, Streets of Laredo (1993). I enjoyed the other 3 well enough. But Lonesome Dove is in my top 12 all-time.
David Baldacci's The Camel Club. He wrote 5 books in this series. It's terrific. The protagonist is a homeless guy, Oliver Stone, who lives in a tent in the park across from The White House. Of course, Oliver is not just your average homeless guy.
Tom Rob Smith's trilogy with Child 44 as the lead. I loved this series. The main man is a young KGB officer in the 1950's who is on the fast track for advancement. However, he ends up developing a conscience. This doesn't really jibe well with the philosophy of the KGB. This series only ended a couple of years ago. He has just come out with a new book, The Farm.
John Jakes' The Kent Chronicles. This is definitely one of my favorite series ever. It's 8 books and starts in the early 1770's and goes all the way into the 2nd decade of the 20th century. The whole series begins with a young man that leaves England for the colonies prior to the American Revolution. This is historical fiction at its very best.
Ken Follett's 2-book series about England in the 1100's and 1300's. I'm cheating a little bit here because Pillars of the Earth really stands on its own. In fact, it's one of my 3 favorite books ever (with Shogun and The Source). Nevertheless, 18 years later Follett wrote a sequel. It's takes place 200 years after Pillars and is loosely based on the families from book 1. That's a series, darn it.
Harlan Coben and Myron Bolitar. There are 10 of these, but only one in recent years. For all intents and purposes, Coben is done. Myron is an All-American college basketball player who gets injured and can't play in the pros. He becomes a sports agent, and each book is a murder mystery. Plus, Myron has one of the best-ever sidekicks, Win Lockwood.
Part 2 will be coming soon to a computer near you.