Really hard to pick just one because I've read so many really good series. So in no particular order with the only similarity that I have read a significant portion of it and I am waiting for the end.
Blood of Eden series by Julie Kagawa, dark, gritty, violent and headed by an main character who will do anything and everything to survive the absolute nightmare the world is living.
Eon and Eona by Alison Goodman, genbending, dragons, swords, politics, intrigue, superb world building. What more do you need?
The Lumatere Chronicles by Melina Marchetta, Low Fantasy YA, politics, war, murder and redemption. Awesome!
Kate Daniels series by Ilona Andrews, superb world building, awesome supporting cast that have personalities and are cardboard cutouts like most of these characters are in other series. Magic, swords, mythology from world religion and cultures, an awesome main character. Curran, hmmm, Curran. Awesome series all around.
The Travises series by Lisa Kleypas, romance crack, awesome heroines, awesome male love interest, well flesh out relationships, hot sex. A-M-AZING!
The Wallflowers series by Lisa Kleypas, romance, hot sex, scandal, bonnets, parasols and top hats. Flat out some of the best historical romances on the market.
Tairen Soul series by C. L. Wilson, in the mood for High Fantasy Romance? High tolerance for cheese? Well, this is it then. Over the top romantic declarations of love, cheese galore, well flesh out relationships, war, blood and creepiness all around.
Well, there you go I couldn't pick just the one.