Leipzig vs. Freiberg vs. Chemnitz Christmas Markets

The Chemnitz Christmas market made 1st place at Germany-wide award in 2014. Chemnitz and Freiberg are close enough to visit both in one day. Leipzig has three different Christmas Markets, and I bet they are all outstanding. (I don't see a point in doing a Christmas market if it isn't outstanding.)

Of the dozens of Christmas markets I've attended, only a few really stand out, and most of these were because they took place in the region where I lived and I knew many of the people in attendance. I wouldn't quite say that most Christmas markets are the same, but in comparably sized cities, most are very similar. So, I would simply choose based on which is the most interesting city.

I have not visited Freiberg, so I can't comment on it. In some circles, Chemnitz (known as Karl-Marx-Stadt during the DDR period) has the reputation of being Germany's ugliest city. Although I think that reputation is rather undeserved, it's no gem of a city either. On the other hand, Leipzig is a delight. It was mostly spared both the destruction of WWII and the most of the hideous architectural tendencies of the Communist period. Most of what you see survives from it's glory days as one of the commercial capitals of the German Empire. It has a rather large pedestrian-only shopping core, and I'm assuming the Christmas Markets mostly take place here. Of the three places you listed, it would be my choice.

What makes a Christmas market special is first of all the location and the atmosphere. The goods on sale are somewhat interchangeable no matter where you go.

From that perspective my vote would be for Leipzig which is an amazing city. Maybe you even can time it to take in an Advent/Christmas concert in the Thomas Church.