(In no particular order...)
Quinnipiac University Poll
"The Quinnipiac University Poll conducts timely and accurate public opinion polls on politics and public policy in Connecticut, Colorado, Florida, Iowa, New Jersey, New York, Ohio, Pennsylvania, Virginia and nationally as a public service and for academic research."
Gallup Polls conducts regular public opinion polls.
"HuffPost Pollster tracks thousands of public polls to give you the latest data on elections, political opinions and more." Pollster focuses on polls that claim "to provide a representative sample of the population or electorate."
FiveThirtyEight provides analyzes statistical information to explain and forecast political events. They also provide pollster ratings by assigning a "grade" to pollsters, which are "calculated by analyzing the historical accuracy of each firm’s polls along with its methodology."
RealClearPolitics aggregates polling data and calculates an average of these.
Pew Research conducts research on a wide range of topics. In terms of the 2016 general election, they focus on "people’s engagement in the election, opinions about the candidates, views of the campaign and voter preferences."
Since 1994, The Hill has reported on the intersection of politics and business, offering objective and in-depth coverage of Congress, the Administration, business and lobbying, campaigns and more.