But "is" refers to me and my preference, not (directly) to my favourite.
Can I not say that my favourite music composer is JS Bach?
If I say that my favourite composer was JS Bach, does that not mean simply that I have changed my mind? Surely it doesn't mean that I was contemporaneous to him, and he remained my favourite until I outlived him?
Can I not say say that my favourite composer was Katy Perry, but now is JS Bach?
Not being persnickety here; my favourite continent is Pangaea. Really!