The Vorkosigan saga by Lois McMaster Bujold: Starts off as a slightly quirky set of space opera tales, the hero being both the Emperor's cousin and a liberal dwarf; but culminates in a set of brilliant character studies - Mirror Dance, Memory, Komarr, and A Civil Campaign - which I would recommend to anyone. (I was disappointed by the more recent Diplomatic Immunity, but hope that the new volume just out will be a return to form.)
Other sf series: the various Doctor Who runs, obviously; apart from that I am a bit pushed to think of any set of four or more sf books in the same series, all of which I have both read and enjoyed.
A Song of Ice and Fire by George R.R. Martin: Tolkien excepted, I find it difficult to care much about epic fantasy. But Martin's series set in a world halfway between Middle Earth and the Wars of the Roses, told partly through the eyes of a family of noble children, growing up in the middle of a civil war which tears them apart, is an exception, though I wish there was a bit more action in the more recent books (and that the next one will come out soon). Really looking forward to the TV series of this one.
Other favourite fantasy series: Pratchett's Discworld, Zelazny's Amber (first half); Le Guin's Earthsea; Gaiman's Sandman; Bill Willingham's Fables; Juliet McKenna. Odd how fantasy seems to lend itself more to series than sf; showing its roots in the sagas?
The Rebus series by Ian Rankin: A relatively recent discovery for me, in a genre that I had not otherwise read much since I was a teenager. Rankin's Scotland is as much a fantasy country for me as Barrayar or Westeros, and he subverts the police procedural by turning his magnifying glass on the structure of society and the nature of justice; Rebus' own flawed character is a mirror for his society in a way.
Other mystery series I used to read: Lindsey Davies' Falco stories set in ancient Rome, which are frankly a bit variable; Patricia Cornell's Scarpetta novels, about a forensic pathologist in Virginia, which I felt got repetitive and implausible as the series went on; and Janet Evanovich's series about New Jersey bounty hunter Stephanie Plum, which stopped being funny after a while. As a teenager I also devoured Agatha Christie, Ngaio Marsh and Dorothy L. Sayers, of which I suspect only the last-named would survive a return visit today.
I am sure there are horrendous omissions from the above, but those are what came to mind.
Day 01 - Your favourite series of books (with more than 3 in the series)
Day 02 - A book that you wish more people had read
Day 03 - Your favorite recent book
Day 04 - Your favorite book ever
Day 05 - A book you hate
Day 06 - Your favourite writer
Day 07 - A writer you don't like
Day 08 - Your favourite work in translation
Day 09 - Best scene ever
Day 10 - A book you thought you wouldn’t like but ended up loving
Day 11 - A book that disappointed you
Day 12 - An book you’ve read more than twice
Day 13 - Favorite childhood book
Day 14 - Favorite male character
Day 15 - Favorite female character
Day 16 - Your guilty pleasure book
Day 17 - Favorite trilogy or tetralogy
Day 18 - Favorite book cover
Day 19 - Best ensemble of characters in a book
Day 20 - Favorite kiss or love scene
Day 21 - Favorite fictional romantic relationship
Day 22 - Favorite ending/climax
Day 23 - Most annoying character
Day 24 - Best quote
Day 25 - A book you plan on reading
Day 26 - OMG WTF? plot
Day 27 - Favourite non-mainstream writer
Day 28 - First book obsession
Day 29 - Current book obsession
Day 30 - Saddest character death