Each state has its own list of best interests factors. No single factor is any more important than the others – they're weighed in total. Some common themes include the child's preference if he's old enough, which parent traditionally cared for the children when the marriage was intact, and whether either parent is likely to interfere with the children's relationship with the other parent.
If you've loyally worked for the same company for years, a judge might consider that a good reflection of your character, and a parent's character is a best interests factor in many states. If you've shown a tendency to job-hop, however, this might indicate to the judge that you have a problem committing yourself to important things. Some states, such as Michigan and Tennessee, look to which parent demonstrates a capacity or disposition to provide for the children's needs, so a parent who chronically avoids working might not be favored in this respect. In Tennessee, judges can base custody decisions, in part, on the available hours you can devote to your children, so a workaholic parent may not win points either.