I am looking for a book series that upon completion will leave a feeling of emptiness, as if I'm saying goodbye to a good old friend, when you realize the good old days were left behind. It's a shitty feeling that only comes after something great, something that left a mark on you.
Riyra series by Michael J Sullivan. You can feel free to stop after the original trilogy, but I highly recommend the full set of books in the world.
I also recommend the Iron Druid series by Kevin Hearne. You get really connected to the main characters.