The other big ones besides tax are welfare and hiring levels. If we can't hire enough people to keep a corp staffed, we won't stay there - I routinely move my corps whenever hiring falls much below 100%. And your national welfare level has a big impact on productivity.
If you can keep hiring at 100%, welfare over 120, and tax at 30% or less, you can easily post an ad on your world forum saying you're looking for CEOs and get some. The better those numbers are, the more interest you'll get.