this one is very hard to answer because basically every character is hard to write >_<
I think the hardest ones are those I do not find compelling, or that are farther from my experience. If you ask me to off-the-cuff a person of another culture, or a person whose value system I don’t know very well, I will do a shit job of it. That stuff needs Prep and sensitivity consultations. Same for, say, a real, believable US high school athlete who isn’t just a jock caricature.
I mostly just cheat by picking areas for stories I feel more confident in telling, so that characters are a little closer to my voice than not. Give me a Shy, Insecure Top, or a Spunky, Gentle Partner, or a Hardass Mom, or — from what I’m writing currently — The Most Ever Dad to Man A Grill.