It depends on your definition of a "good master".

Medosa is one of the best servants in this respect, as she faithfully follows her master and remains unaffected by her emotions.

Most of the stronger servants aren't like her — they tend to be influenced by their emotions if their master's personality conflicts with their own.

Well she was summoned by Sakura without a catalyst, so they won't have any personality clash issues, and Sakura is one of the strongest masters in the war in terms of providing magical energy. So no one. Sakura is basically her ideal master.