あなた don't exactly decide. At least in my opinion. Anyways, most of the time your favourite characters can grow on you, others あなた find あなた start to like over time, または in some cases a character あなた grow to like even though あなた 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 もっと見る and his personality. He then became my most favourite ヘタリア character. I absolutely just 愛 Prussia and I admire him. He's utter perfection in my eyes.
link[/u]