I have only a mild idea what an editor does. I guess in the case of an anthology, one can point to both the actual editing and also the selection of stories included. I'm really just feeling my way here, and I have a nasty feeling that I may make some mistakes with these later categories. All five candidates have been nominated before, which gives me an odd confidence that I can't really get this one wrong...
Neil Clarke:editor of Clarkesworld and Forever. Oh - and he's a very clever man, carefully detailing all the stories that he's edited that are short-listed for awards. A whole lot of my favorite stories of the past year are on this list, a fact which has just moved him to top of my list. He's been nominated for this Hugo before, but has never won.
Ellen Datlow: I've been reading works edited by her for, literally, as long as I've been reading in the field. I love the short stories she listed and think she did a great job of selecting them. She has previously been nominated for a Hugo in editing, and won multiple times.
Sheila Williams: looking over the issue of Asimov makes me understand just how much I like her selections of stories, too. She's got multiple Hugo nominations in editing and a recent pair of wins.
John Joseph Adams: editor of Lightspeed, which is another source of great stories. He has one previous nomination.
Jerry Pournelle: editor of There Will Be War Volume X, which I've mostly read by this point as I waded through the short stories and novelettes. Sadly, I don't see this volume as evidence of editorial skill. The e-book itself has multiple kerning errors. I also think the stories are second-rate or worse.