Should be 那位明星 also, since it's a person
个 is fine for people in almost all circumstances, unless you are being extra polite, like when a restaurant asks how many people are in a party: 几位?.
In this case 位 sounds weird to me.
谢谢, 可是 为什么?
by "that celebrity" i mean rihanna
Wǒ zuì xǐhuān nàgè míngxīng
So the adverb goes before the verb?
Literral translation?: I the most like happy that bright star
Wow, so it's spelled 明星. I thought it was spelled 名星 as in "famous star", given that "famous" is 有名. Although apparently 明 also means "to know". And they are both pronounced míng. That's a bit confusing 囧.