Oh well, This was hard. I have read so many INCREDIBLE series that it is impossible to pick just one. At this very moment, Stacia Kane's Downside Ghosts is the series most on my mind.
I really should hate these books. The heroine, Chess Putnam is a drug addicted waste with no self esteem or worth. She is truly f*cked up. The hero (who's name is Terrible.. if that tells you something about him) is an ugly beast of a man who is the enforcer/bodyguard/hitman for the local drug lord, Bump.
This is a dystopian tale focusing on magic and ghosts. It was proven that after death your ghost goes to an underground city. This city is run by The Church Of Real Truth, in which Chess works for and is a debunker. This city is supposed to be an ideal place of peace and happiness for everyone in the afterlife, but Chess, because she works for the church, has seen it and knows it is not all its cracked up to be. She hates her life, but is more fearful of the afterlife. She is a woman without any hope at all. Chess and Terrible have both had completely awful lives full of abuse and desperation. They do not get together in the first book and the relationship is rocky in the proceeding books but OMG I LOVE Terrible!!
There is a whole downside lingo in these books that is just brilliant. I did the audio on these as they are available in my public library. Bahni Turpin is the narrator and she is perfect for this series! I would highly recommend the audio as the downside lingo is sometimes hard to get through.
Some of my other favorite series are:
Pennyroyal Green by Julie Anne Long
Maiden Lane by Elizabeth Hoyt
Spindle Cove by Tessa Dare
Spymasters by Joanna Bourne
Drakes Rakes by Eileen Dreyer