Governors. That’s why we’ve had a grand total of just three Presidents who were elected to the Presidency directly from the Senate (Warren G. Hardin in 1920, John F. Kennedy in 1960, and Barack Obama in 2008). The executive experience that Governors have gives them a real advantage over legislators when they are campaigning for the Presidency. Now, there is no job that truly prepares you to be President. Nobody genuinely understands how to be President until they have actually been President. Serving as Governor is about as close as it gets, though, especially in large states or states with a strong chief executive. That executive experience allows Governors elected to the Presidency to hit the ground running because they know how to manage an executive administration, albeit on a much smaller scale.
Senators — no matter how high-profile they might be — are still just 1 of 100 and it’s easy to get lost in that shuffle. However, the reason why it is so hard for a Senator to run for President is often overlooked: a Senator spends his/her term voting on issues that are often controversial. Governors don’t have a voting record that they have to explain or defend. A Senator’s resume is often nothing but their voting record, and Senators often have to make decisions that might be beneficial to their constituents, but which hurt them everywhere else in the country. That can really come back and bite them in the ass when they go to other parts of the country and ask for support.
For a more in-depth, scholarly look at Governors and the Presidency, I highly recommend a book written last year by Saladin M. Ambar, How Governors Built the Modern American Presidency.