How to reject counter-offer politely?

I work in Company A and I'm kind of the star there (I feel that mainly due to political reasons). Recently I received offer from Company B and I decided to accept it. HR department in my current company push me to review my decision and propose them conditions under which I would like to stay at company.

I feel that offer from Company B is much better for my career (project is more interesting, company is also a better suit for me), but I don't want to burn bridges. How can I respond?

Been in a similar situation before, and the best advice I got from my mentor was to tell Company A that you're leaving because the opportunity at Company B is something you really want to explore. When I was in the situation, Company B was a completely new industry (fashion) than the one I was currently in (consumer packaged goods). By emphasizing that it's the opportunity to learn something new rather than compensation or title changes that's informing your decision to leave, you put yourself in a more positive light because any good manager/company should want you to pursue things that let you grow so they should understand leaving because for a new chance to learn.