If Rhaegar killed Robert at the Trident, then he'd effectively take the edge, but it would take a while longer to seal a victory. Stannis would still hold out for a long while at Storm's End since he's unlikely to bend the knee to the murderers of his brother. (even if he didn't like Robert) The Vale-Riverlands-North Alliance is still a major hurdle and the only way to solidify a conclusive victory would be to capture or kill Ned/Hoster/Jon to force the regions to surrender or sue for peace. The Iron Islands and Westerlands would pick the winning side and start harassing the Riverlands and the North. Now would Rhaegar be a better king for the realm? Considering the low standards placed by his father, Robert, Joffrey and Tommen...he'd be a better king for the realm by leaps and bounds. Probably even a great king by himself.