We can write 'baka' like ばか, バカ, or 馬鹿 in hiragana, katakana, or kanji. Among them, バカ(katakana) is the most common. The choice of character does't change the core meaning, but the nuances may differ a little depending on the writer. For example, ばか in hiragana looks rather softer than others.

actually We usually write ばか　in Hiragana.

Good question. I hope you study ”aho（あほ、アホ、阿呆）” together. It's almost same meaning with BAKA. It is used in Kansai Area, like Osaka, Kyoto. It can be witten in Hiragana, Katakana, Kanji. I think Katakana is most popular to write AHO. http://www.geocities.co.jp/Hollywood/7779/ahobaka.html This is a map to resurch area of BAKA snd AHO. Red ● is "AHO" Area. Blue ▲ is BAKA Area.

The main reason I thought バカ is common is that both google and bing gives much more results for バカ than ばかor 馬鹿. But the choice of character really depends on the writer. Thanks for correction!