I liked Waid & Samnee's Black Widow series.
I liked most of Edmondson's & Noto's Black Widow run before that (though the last two issues of Last Days tying into Secret Wars were a waste)
And going back a bit, the Marjorie Liu & Daniel Acuña five-issue arc (from 2010) was also great.
She also was featured in Ed Brubaker's Winter Soldier run (though Brubaker really messed with Widow's mind in that). What are some other worthwhile Black Widow runs from the past few years?