Hands down for me is Ralph McTell. He has touched on nearly every emotion, but includes great historical context in some songs as well.
He also writes songs that I think are uniquely his, which to me is the greatest compliment, not just of a songwriter, but as a muscician as well. His way of writing about subjects most people wouldn't touch (Michael In The Garden, for example), is what really sets him apart. There are others, for sure, but none are as complete as Ralph is for me personally. Some other names though, just for fun would be Richard Shindell, Carrie Newcomer, Steve Tilston, Iain Matthews and Thea Gilmore (Thea arguably is rock, or rock-pop, but I still think of her as a singer songwriter).