u don't exactly decide. At least in my opinion. Anyways, most of the time your favourite characters can grow on you, others u find u start to like over time, of in some cases a character u grow to like even though u didn't like them at first. Some times characters can become your favourite right away. -smiles-
The way Prussia became my most favourite character was an over time thing. At first I actually didn't like Prussia. I thought he seemed loud and kind of obnoxious. After awhile, I started to see his character meer and his personality. He then became my most favourite Hetalia character. I absolutely just love Prussia and I admire him. He's utter perfection in my eyes.
link[/u]