"as a favorite" is correct and usable in written English.
It is used to describe something that a person likes or enjoys the most. For example, "He enjoys ice cream as a favorite dessert."
He began the presidential campaign as a favourite.
The 1990 film Ghost cemented Swayze as a favourite.
Parallel with individualism arose, as a favourite humanistic theme, the idea of the dignity of man.
But he is also widely touted as a favourite for the Republican presidential nomination in 2016.
Along with older manufacturing, clean technology is emerging as a favourite direction.
Many commentators are tipping Ghanian Cardinal Peter Kodwo Appiah Turkson as a favourite.
"Not that we were as a favourite beaten by a club," he said.
The tradition of dressing as a favourite character makes World Book Day feel extra-special.
In that time she did much to promote it as a favourite public destination.
He arrived in Athens confidently as a favourite to win Olympic gold.
Blink and you missed the team that entered this series as a favourite to many.
