It's hard to narrow down to just one. There have been a lot that have influenced my writing, at different phases of my songwriting development.
So, I suppose, in chronological order, it would look something like:
•Jay Farrar (Uncle Tupelo)
•Paul Westerberg (The Replacements)
•Bob Mould (Hüsker Dü)
•Nick Lowe
•Hank Williams

But, for my favorite that hasn't OVERTLY influenced my writing (although I'm sure that he's in there somewhere):
Smokey Robinson