Tuning SVM C parameter
Essentially, you only want to look at your test accuracy. The training accuracy is only relevant in comparison with the test accuracy. In your situation, it seems that your test accuracy is roughly the same across the board beside a slight peak around 0.620. So I would pick this as the best value for C.

My answer is related to the one I gave on this post, here is a summary:


  
  Overfitting is not when your train accuracy is really high (or even 100%). It is when your train accuracy increases and your test accuracy decreases
  is low.
  it is not abnormal that your train accuracy is higher than your test accuracy. After all, your model has an advantage with the train set
  since it's been given the correct answer already.