It is just an idea. There could be once a month voting for "best" member of the month. It would be just a way to say thanks for all the help. Every member could for example give 3 votes for others (not possible to self vote).
Criteria for giving a vote is up to personal preference. Someone will vote for the member that was most helpful to him, someone for who is writing good quality answers, etc... Over large number of voters this personal preferences give good average.
The big question is: with what purpose should we do that? Features like that reflect popularity, which is rarely a good criterion for someone's real value. Even the voting system and the reputation represent only a small area of being useful, and then only mean an indication. In short: a "member of the month" -feature brings more injustice to hidden value than it does to obvious popularity.
I think one of the biggest attractions of AU is that we focus on result and quality, rather than celebrating individuals. It actually feels as a relief in the jungle of screeming popularity contests. Let's keep it that way.
In some ways, there is something already here to show who is doing the 'best' this month... (automatically based on reputation/voting/editing):
But please remember it would be better to be helping :) , not just competing against other people, or doing a million edits to everything, or spamming random answers just for upvotes etc. As @MarkKirby said, you can award a bounty to someone if you think they have done useful answers.