Nash at his peak was the better player. I think there's an argument that Hall may have had the "better career". Part of it comes down whether you value one of the "weakest" Hart wins over a Rocket Richard in a pretty tough field. Typically, i'd lean Hart > Rocket. But in this case, i think it's more debatable.
The other aspect is that i think Nash was the bigger, better, more responsible player...and has some bigger international accolades to his name, particularly as a standout on a Gold Medal Olympic team. He was also more consistent throughout, while playing on some bad teams without a lot of help most of the time through his prime.
However, there's also the argument that Hall played more "meaningful" NHL games. Though to me, that's really more of a "team argument" rather than anything to do with the player.
So on balance, i go with Rick Nash. But i can at least see the argument for why someone might plausibly go with Taylor Hall.