Best Single Arc Series

[This poll is now closed to more nominations, but you can go vote!]

What is the best single arc series?

Even as you pop over and vote on our Best Magic System poll, let us move ahead on our extremely late February poll so that we can try to catch up to a poll each month.
We do a lot of "Best Series" polls here at Writing About Writing, but today we're going to look at a very specific kind of series. Many "series" tell several stories in a single world. Series like Dresden or Discworld. But some series are really a single story spanning multiple books.

While one or all of the books (especially the first one in many cases) might stand alone, the series itself is really a single work.

What is the BEST series that is really a multi-book single story?

The Rules

1- As always, I leave definitions up to your best judgement because I'd rather be inclusive. If you feel that Harry Potter counts as a single work, even though clearly each of its stories (especially the first three) are stand alone works, I am not going to quibble over the nomination. If there is a specific run of three books in Discworld that you feel tells a SINGLE cohesive story, you can nominate that. But the point here isn't to bend the rules to get a series you like on the poll, (we do plenty of polls–there will be other chances to shout out your faves); the point here is to really nominate the best multi-book single story.

2- You may nominate two (2) series. (Remember that I am a horrid and unyielding power hungry monster* here at Writing About Writing. To encourage reading and reading comprehension I will NOT take any authors beyond your second nomination.) If you nominate more than two books, I will only take your first two and consider any beyond that to be pre-seconds for a future nomination.

3- You may (and should) second as many nominations of others as you wish. No series will be going on to our poll that doesn't get at least one second.

4- The series must be done. Otherwise we don't know if it's got an ending that really lives up to "the best single arc."  So, for example, no Song of Ice and Fire.

5- Please put your nominations here. I will take books nominated as comments to this post on other social media; however, they may not get the seconds you need because no one will see them.

6- PLEASE NOTE: If we end up with an unwieldy number of nominations, the first split I will make will be the science-fiction, fantasy divide. If you feel your series is in any way genre bending or ambiguous, please include which way you would to see it go should the split occur. Otherwise I'll use my best judgement.  And while I don't anticipate any series that aren't either one or the other, if they do show up, I'll do my best.