What is the No. 1 thing that distinguishes a superior employee from a typical one?

The one who always keep on improving himself 

commitment.

Always exceed their agreed performance measures.

Ability to look at the bigger picture

Motivation. If the employee isnt motivated and interested in his or her job, then no matter how skilled or professional they are, they will lack the ability to shine.

Attitude is the most important personality trait for employees.

It is of couse "Professionalism".

UNIQUENESS

(his / her ability to stand out from the crowd by having the following qualities in him / her - which means, a whole package of qualities in one such as strong commitment, hardworking, sincere, quick learner, focussed, talented in many ways, multi-tasker, accurate, good interpersonal skills, good communication skills, meticulous etc.....)

Employee with with great care for details, whilst not losing sight of the big picture

There are multiple indicator of high performer which could be;

1 - Resulted orientied.

2 - People orientied.

3 - Effective communication skills.

4 - Self motivated.

5 - A Good learner.

6 - Creative & solution provider.

There are other traits too but the above one are what most of the time employers looking for.

A superior employee is result-focussed, not effort-focussed, an example in integrity and behaviour for his teammates and subordinates.