Definitely Doughty and Richards. They are the more proven combo. Doughty isn't scoring as much this year (more due to LA's style rather than a drop off in his play) but he's scored 60 points as a defenseman before and is top-5 in the league. Richards has been a bit inconsistent this season but he's still good for 60+ points per season as well as adding some hits and PIM. Both of them being on a rock solid LA team for the next several years will also help your plus minus a lot.
Stepan may turn into an All-Star who's good for a point per game but he's not there yet, and worryingly his play this season has dropped off from last. He also doesn't have the grit that Richards has so you're losing out on hits and PIM. Franson is playing well for Toronto but he's been maddeningly inconsistent at times.