1. Okay, then prepare a one-page template which consists of five questions and ask each of your department heads to fill it out quarterly. Based on all their comments, choose the nominee for the Best Employee Award.
Here is a template that I use in my company for Peer Recognition Award.
2. If this doesn't work, then a simple way to execute this is—let each department head choose a member from their team (mentioning the reason for nominating the nominee and their key role played) and email their name to HR. Now, HR and management can decide on it.
3. If this doesn't work—quarterly, call everyone for a meeting (including management) and let each member convey what they have accomplished in the last 3 months (give 5 minutes each). Let management announce the name.
4. If this doesn't work—ask your employees to send a timesheet to HR on a monthly basis, which includes a synopsis of the work they have performed for the entire month. Based on that, choose the Best Employee.
5. If this doesn't work, then HR should conduct a survey to find out the star performers either by interacting with the department heads or by interacting with the employees themselves. This includes both performance and character. HR can easily come to a conclusion on whom to nominate.
The most important thing is that HR should always be ready with an answer to face questions from the employees because everyone assumes that they have done their best and they are the best. Everyone will be curious to know on what grounds you have chosen a nominee and why their name has not been included. When we choose an employee as the Best Employee, then we should be able to explain how this particular employee is different, best, or unique from others, as well as why he/she was eligible for this award.
Good Luck... Let me know if you have any queries.