I know we can use list of employees, but I'd like to know which is preferred or more correct: employee list or employees list?
"Employee" here is a noun adjunct (a noun used as an adjective). Noun adjuncts tend to be single (raisin bread, rose bush, bike path, etc.) because adjectives in English don't have a number.
See examples and explanation here: http://www.englishcorner.vacau.com/grammar/rules/nounadj.html
So, Employee List would be standard.
I would go with Employee List (or Employee Directory). It is far more common to use the singular in this case. (You can Google both "Employee List" and "Employees List" to prove it to yourself.)