Best companies for a scrum master

Depends, on basically everything.

Any company that is doing Scrum.  

Your question is really vague.  If you can provide a bit more information on the background for your question and why you asked it as you did, you will get much better answers. 

A company that is doing scrum or wants to implement it.

I myself am in the IT department of a communications company, because my roots lie in IT and I think it is there that I have the most added value and feel at my best. 

A company that knows the problems it wants to solve with Scrum, but doesn't necessarily know how.

One where you are trusted to carry out your role as Scrum Master, including when that touches on areas that are uncomfortable for the company.

Perhaps you should reverse the question a bit. For which kind of companies am I ready.

what companies would be the best to work for as a scrum master? 

May be you need answer 'why you need to work as a SM', rest you will come to know on your own. Good luck !! ;-)

I've seen a lot of gaming companies that are exceptional.  Really big companies have a tough go of using Scrum due to legacy bureaucracy.  Smaller companies are frequently less constrained (a real generalization).