I have painted a few so far, along with a few cars in the past. I used flat, gloss and most recently, semi-gloss black.
I like the look of the semi-gloss black the best. Flat just looks too flat and everything seems to stick to it, gloss is a bit too shiny for a grille and gathers water spots real easy. The semi-gloss just seems right, not too shiny, not too dull but still has a shine to it. It's almost closer to a factory look as well aside from being black instead of that grey.