What was the most successful newspaper in the colonies?

The total number of newspapers in the American colonies was a very small number. There were 2 in Massachusetts, one in Rhode Island, one in New Hampshire, 2 in New York, 2 in Pennsylvania, one in Connecticut and 1 in South Carolina.

All these newspapers were weeklies and few lasted for more than 10 or 20 years at a time. These journals in the 1760s and 1770s were mainly political in nature. The front page of most of these papers had one or more editorials that dealt with the issues of the days. They were also used to inform the public about decisions made by their colony's government. They discussed the comings and goings of officials on ships, the happenings in England's parliament, and did a small amount of commercial advertising.

Typically they were two main operations with a noted political leader owning and sometimes publishing the 4 page newspaper. The life of the newspaper was almost always dependent upon the life of its publisher. It was not unusual for a paper to cease publication when the owner lost interest.

The newspaper as a commercial undertaking did not really come into being until the mid-19th Century. The New York Times was founded in 1851, the Boston Globe in 1872, the Washington Post in 1877, the Philadelphia Inquirer in 1817, and the Cleveland Plain Dealer in 1842.

Those 19th century newspapers were a radical change in form from the 18th century newspaper and so success of one newspaper over another during the colonial era is both a relative and subjective idea.