I can't claim my choice is "the best" but I know it works well. I have a handful of these JAWCO brand (made in USA? dam straight....) thread repair dies in our typical metric sizes on the 911. They're tiny size for getting in tight spaces.
https://www.mcmaster.com/2667A453/
https://www.mcmaster.com/catalog/128/2815/
Use some lubricant like penetrating oil (Gibbs, Kroil, 50/50 mix of auto trans fluid & acetone, etc.) to help dissolve the rust and it also reduces wear on your tool. You can use a 14mm deep socket to run the tool up and down the threads. Then clean with solvent and put some high temp anti seize paste on the studs.