Uh… Tom Swift Jr.? 
Yeah, Sherlock Holmes.
And several of the Dr. Doolittle books.
ETA: And how could I forget?! Asterix and Obelix!
I’m an omnivorous reader, without much taste, so I’ve read most of the popular series in most of the genres. I just finished binge reading the entire Prey series by John Sandford, and IMO he makes Lee Child’s books look like junior high essays. But I guess I like Conan the best of all the rough, tough, nobody-messes-with-me characters — not just the original RE Howard stories, but many of the later ones, especially the Robert Jordan and John Maddox Roberts novels about Conan. I never heard of John Puller until I read this thread, so I guess I’ll give him a try.
For military adventure, I’m one of the few who doesn’t especially care for the O’Brian books, although I like the Hornblower, Bolitho, Lewrie, and Harrington books, (but I skim large portions of the Harrington books when Weber goes into excruciating detail on imaginary weapons systems). I also like all but one of the Vorkosigan books (Cryoburn sucked, IMO, but she bounced back with the Vorpatril book).
But if the winner goes to the number of books read, then it has to be Remo Williams, who’s featured in well over 100 Destroyer books.
The Bernie Gunther series by Philip Kerr. Noir detective in a Nazi setting, pre- during - and post - WW2.
I was just going to say exactly this- they are very smart, funny and interesting mysteries. Love his relationship with Louise (better in the book than the series but loved both).
And ditto to what Wallaby said- Harry Flashman!
Most of my favorites have been mentioned already, but I can think of a couple more.
Steve Hockensmith’s Holmes on the Range series is great.
Robert Galbraith (AKA J.K. Rowling) has the Cormoran Strike novels. There are only two of them right now, but the third is due out in October and can be pre-ordered from Amazon already.
I read a lot of series when I was younger, including Agatha Christie and Burroughs (Tarzan and Barsoom). I enjoyed them then but I don’t think I would be so interested now.
More recently I enjoyed all the books I could find by H Rider Haggard that had Allan Quatermain as a character. I don’t know if that counts as a series, though.
As I’ve posted elsewhere, I am currently reading all 75 Maigret novels by Simenon (in translation). I am now on #68. Sometimes I take a break and read something else in between. But Maigret is among the most fully realized mystery-genre characters that I have come across. He is simultaneously predictable and surprising; he is perfectly ordinary in his daily life, and extraordinary in his abilities. That’s the way I like them. Almost no violence or “suspense”, just good sense and observation. And excellent writing, which always helps.
“Little Britches” and its sequels by Ralph Moody, detailing growing up in the West in the early part of the 20th century. Quite excellent.
Unfortunately, the series has been adopted by fundamentalists. Even though religion plays virtually no part in the stories, they are apparently held up as a good, wholesome, example of a life well lived. This alone might be enough to ward off readers, which is a pity.
Jasper Fforde’s Tuesday Next books.
Joshua Dalzelle’s Omega Force series. I just went through all 7 of them in the past month.
It’s “Preston” and Child. And yes, the Pendergast novels are great.
Elvis Cole stories by Robert Crais
And Conan by Robert E. Howard (only the Howard stories, though)
Does it matter which character is recurring? I recommend O’Brian’s Maturin and Aubrey series.
I liked the before and during WW2 books, but stopped reading after the first post-war entry.
Studs Lonigan and James T. Farrells Washington Park/Danny O’Neill pentalogy
If you’ve read the Robert Jordan and John Maddox Roberts books about Conan, and didn’t like them, then de gustibus. If you haven’t read them, seriously, give them a try. IMO they are both at least as good as Howard. It’s notable that both have since done very well with characters of their own creation.
I agree that the L.Sprague de Camp and Lin Carter stories that were included in the old paperbacks did not deserve to be in the same books as the Howard stories. And some of the other authors of Conan paperbacks, whose names I have mercifully forgotten, were execrable.
Just a one and its sequel, but I absolutely loved the Murphy/Cochran GrandMaster and High Priest books and really really REALLY wanted a third. Unfortunately, world history kind of got in the way.
I also loved Zelazny’s Amber series (both of them) and wished Corwin could have come back. And I like Feist’s Magician trilogy and the two after, but decided not to follow the rest. My friend did, and said he got a bit confused when characters started using their GGG-grandsire’s name (in honor thereof). I didn’t bother with Wurtz’ corollary series about the Empire’s machinations.
—G!
As fond as I am of Andrew Offutt, his Conan books were clearly about some other character entirely. He was writing detailed intrigue and plotting stories, with lots of intricate machinations, whereas Howard’s Conan was more straightforward than that.
(Other writers went too far the other way, making Conan too dumbed-down. The bloke could conspire – he just couldn’t – or wouldn’t – do really detailed “triple-think” Frederick Forsyth style scheming.)
John Jakes’ “Brak the Barbarian” stories were ghastly. I’ve never read any of Jakes’ serious stuff, after suffering through those hellacious piles of steaming ordure.
Me too! We’re a rare breed, in that most people who liked the first series didn’t like the second, but I felt the second was pretty darned good, and a worthy follow-up to the first.
A number of readers seem to be decidedly un-keen on Cryoburn. I’m just curious – what is it that people find so awful about it? While I have some issues with a couple of the earlier novels starring Miles V.; I greatly like Cryoburn. It’s “different” in various ways – a new setting, not featured elsewhere in the series; and maybe less-horrendous villains than on other planets (although their doings can get you just as dead) – but I personally take pleasure in its differences from other “Vorkosiverse” milieus.
Well although they have a huge cast of finely crafted recurring characters Ed McBain’s 87th Precinct stories really center on Steve Carella.
Steven Brust’s Vlad Taltos series is one of my favorites.
I love culinary mysteries. My favorite is Susan Witting Albert’s China Bayles.