I want to take a weighted poll but not sure how to do it.?

Let's say there are 150 employees in an office and there's a chart w/ the names of every employee posted on the conference room wall, and every employee is asked (they are not mandated -- they are just asked) to rate each employee's performance on a scale from 1 - 100 (100 being the highest/best). At the end of the polling, a list will be displayed showing every employee's average score in order from highest to lowest (and the winner wins a steak dinner).
Because not every, single employee will vote and not every employee who votes will vote for all 150 employees (they may just vote for their close friends) a simple average score will not be accurate. (Ex. if there's an employee who only receives one single vote but that vote is for a 100 -- the highest rank -- then he/she will have a 100% average. Whereas if another employee receives votes from every other of the149 employees but is given an average of 99% he/ she will still lose the contest, which is unfair.)How do I make this fair?

I'd average the top n scores for each person. If a person doesn't get rated n times, they get zeros for their missing scores. I'd use n=15 because it's 10% of your employees, too large for one or two people to skew the results & small enough that most people should get at least that many ratings. You could also do something like take the top 15 scores, throw out the high one & average the rest, average all the scores throwing out the high & the low, etc.

The key is to set a minimum number of ratings, then arithmetic mean or any other formula you choose on that set should be fair.