What is the best way to motivate teaching assistants? I am head TA in a 300-student class, with 12 other TAs.  Currently I do the following things to motivate my TAs:


We have an internal Slack channel called #shoutouts where I mention TAs every time I notice them going above and beyond.
I post Piazza statistics to the Slack channel every week so that we can see who is posting the most, and then rank TAs in order of how much they posted (currently, a lot of students say that Piazza is very helpful)
I post grading statistics to the Slack channel so that people can see who grades the most (and least) submissions, and what is the average grade they give.
I have a feedback form for students to give feedback to the course staff, and I send a message to the #shoutouts channel every time a student says a TA has been especially helpful.
I tell TAs they are getting evaluated by the instructor (which I think a lot of the TAs didn't know).


However, I have been told (by someone not involved with the class) that I am "overquantifying" things.  Also, even with all of this, there are four people who I would describe as "bad TAs" (and five people who I would describe as excellent TAs, and three people who I would describe as good TAs).

What are your best ways of motivating TAs?
I think the best way to motivate a person in academia is to give them a bit of job security, some funding which enables them to follow their interests, and a great team to work with. I am not sure if you can realistically give "your(sic!) TAs" any of this. 

In absence of solid motivating factors, you are trying to engage the team in some form of internal competition, which may motivate some people and completely put off others. For example, I personally would be puzzled as to why should I compete for some statistics or evaluations which look rather meaningless unless it has any effect on my promotion or chances of future employment. 

Most people I worked with responded well to:


smart goals
positive example
cutting the crap (including crap meetings)


In my experience, teaching is rather a pleasant activity and there is a lot of gratification in the  process of working with students. An excessive amount of paperwork, bureaucracy and management can only spoil the fun.