Sitting down, but I used to stand until I was shamed on the internet for it in a similar thread. I legitimately had no idea people could wipe sitting down until an internet thread. I didn't know about this whole "lift your butt cheek off the seat" business. I still probably do something weird.
Pooping is one of those deeply personal things that anything that somebody does that's different than how you do seems weird, foreign, and gross, even though it's all just pretty normal.
As someone who has done both, wiped sitting down and standing, there's no cleanliness difference. Your butt cheeks don't smoosh poop around or anything,
I wish we had a bidet. When I add another bathroom to the house, I'm adding one.