One of my favorite websites is that of RealClearPolitics. They keep track of all the most recent polls on a wide variety of political subjects. And recently, they started keeping track of how Hillary Clinton (the most likely Democratic nominee at this time) stacks up against the leading Republican possible presidential nominees.
The charts below show their findings of the most recent polls. The polls are listed from least recent on the left side to most recent on the right side of the chart. Probably the most important figure is on the far right side of the charts. That is the RealClearPolitics average of all the most recent polls -- and in the last couple of elections, that average has turned out to be closer to the actual outcome than any of the individual polls.
It's still a long time before we even know who the party nominees are going to be, but right now Hillary looks pretty good when paired with any of the leading Republican hopefuls.