Plague marines are a must. Really hard to kill and fit the fluff. If your into fluff, Typhus works well, along with a demon prince of nurgle. I personally use vindicators but just be careful as people know that they pose a threat and want to get rid of them quickly.
"In the embrace of Great Nurgle, I am no longer afraid, for with his pestilential favor I have become that which I once most feared, Death."