I do love Katakuri but I think taking personal bias aside, Doffy is the better written of the two. Doffy has been planned since very early in the series while Katakuri was more recent. Naturally, this leaves more room for Doffy's character to develop, build hype and mystery. Something that I found fascinating with Doffy before he really got involved with the plot was his allure - the pink coat, his psychotic expressions and arrogant nature. Even without being told anything, he was a character that immediately stood out. On the other hand, Kata's introduction was less impressive to me. Gotta revisit the panel but iirc, it was Bege warning Luffy to be careful of Katakuri which is significantly more underwhelming.
Even by comparing their backstory, Doffy's past of being a fallen Celestial Dragon and having to survive with his brother, killing his father really builds to his character more than Katakuri's past. So even if I like both of them, Doffy blows Kata out of the water