Who is best suited to become a scrum master?

It’s a basic question – who can become a scrum master? Is it going to be a developer, a tester, a project manager, a documentation person or a line manager? To be honest, it’s a topic normally influenced by who the deciding person is and why type of a leader is he / she. In a test service providing organization, my vice president was overtly political and will put his close group contacts in key position and this effectively overrides the fact that he / she could have any proficiency J. In another development organization, there was a trend of developers and that too high performing ones becomes a scrum master. In one particular team, a project manager from previous organization had become a scrum master as she wasn’t interested in people line and she will not fit into a product management line. One of my managers was a high risk taker (to the extent of being called a gambler) that he chose to make a PM as a scrum master of multiple teams and also overloaded the colleague with multiple other responsibilities.

Inspite of the preferences and the logic hijacks, who is best suited to be scrum master? There are some specifics like the ones mentioned below which is normally seen.
1.    Skills required for scrum master has no mandatory connection with what the technical proficiency of person but certain skills are good to have for some teams. Foe g. if a product development team has a team of developers using some proprietary technology and the deadlines are very close by, a rampup time in technology will be additional risk. So in such cases, it preferable that scrum master has a bit of background on the technology. However, its not required to be proficient in the technology.
2.    Scrum team is cross functional team and the scrum master should be comfortable to handle concerns / issues of all team members and should be able to comfortably drive the individual streams.
3.    Non dominant attitude : I am often insistent that scrum master is a slave leader. So, any misunderstanding / passion towards leadership is a risk. Its often desired that project management experience is there but should in principle be able to adher to the principle of scrum.
4.    Passion for lean and scrum and its various practices.
5.    Should be able to improvise and sustain the value coming out of system, and should be recognize, visualize and remove the waste.

However, to summarize, its really not required that only a PM can be scrummaster. Any one could be a scrum master.