I am only going to include authors from whom I have read multiple series.
Lisa Kleypas, she writes romance crack, especially her Wallflowers series and the Hathaways Series, are really awesome. And for Contemporary there is the Travises Series. She writes well fleshed out romances and lovely characters.
Ilona Andrews, I didn't like their On the Edge series but their Kate Daniels series is seriously awesome and so is their forage into self publishing with Clean Sweep. You can really tell these two are growing in their craft.
Melina Marchetta writes about the human nature, about forgiveness and understanding and family.