Maybe it's qualifications. Or experience. Or hard skills. Or soft skills. Or work ethic, or leadership ability, or cultural fit. Or maybe it's a combination... but depending on the job, when you're selecting the right person to hire or promote, one attribute is more important than the others.
But here's another way to look at choosing the best person:
Always select the person who wants the job--not the position.
Don't tell me there isn't a difference. Years ago I had an opening for a shipping supervisor. We decided to stay internal. One candidate was clearly better than the rest: Had been in the department for over a decade, great skills, possessed a broad range of shipping and distribution experience, was excellent at training new employees... he was great.
Plus I was glad he was the best candidate because it helped send the message that, even though the company had just changed hands and I had recently been hired, I valued the experience of current employees and wasn't just going to bring in "my guys." Win-win, I thought.
I was wrong. He was terrible. It turned out he wanted the job because he was tired of sitting on a forklift and wanted to sit in a chair. He was tired of scheduled breaks and lunches and wanted to set his own schedule. He was tired of taking directions and wanted to be the one who gave directions. He was tired of punching a clock and wanted to come and go as he pleased.
He didn't want the job. He didn't want to motivate, inspire, lead, manage, discipline, improve, optimize, develop... all the stuff that comes with doing a leader's job. He wanted a position. He wanted what he saw as the perks of the position. He felt he had already paid his dues, even though dues get paid, each and every day, and the only real measure of a person's value is the tangible contribution he or she makes on a daily basis.
He wanted the position, not the job.
And that sucked, because I didn't need a Shipping Supervisor. Shipping Supervisor is just a job title. What I needed was a guy or gal who loved getting product out the door. I needed someone who wanted to be in charge because they wanted to have greater impact how quickly and accurately we got product out the door. I didn't need a person obsessed over a title. I needed a person obsessed about creating an outcome, day in and day out.
Sounds obvious, I know, but in this case it was only obvious in hindsight. During the selection process I focused on the past. I focused on what he had done. I didn't focus on the future, on what he wanted to do. I focused on qualifications, not on the initiatives and projects he had in mind, and not on his motivations and aspirations and goals. I focused on what he had done, not on what he planned to do.
In short, I needed someone who wanted to do the job. I needed someone driven to excel at those tasks so they could make things happen. I didn't need someone who wanted the position because they wanted all the "stuff" that came with the position.
And that's what you need. You don't need a Director of Sales; you need a person who loves helping other people sell more. You don't need an Engineering Manager; you need a person who loves creating new products. You don't need a Supervisor of Whatever; you need a person who long ago made the choice that their happiness comes from someone else's success and who thrives on working through other people to get stuff done.
You need people who want the job because they want the responsibility of making things happen. You need people who want the job because then they can be even more successful at what they do well, can help others be more successful... people who want the job because they want to do the job--and the title only makes it easier for them to do that job.
Always select the person who doesn't care about the position. Even if that person is less experienced or less skilled, his or her motivation and drive for doing the job will quickly make up for any shortcomings.