Inverted y and default x!! When I first started playing I hated the inveres and would only use the defaults, but after a while, I just got so used to the inverted y with the default x that it practically drives me crazy using anything but!!
Pretty Creative, I kinda thought of it as yanking on their hair when i was little. For flight simulators I figured it as like a hangglider, like pulling down, turns them up, you're pulling them towards you, but they kindaof just go up... like to tie a rope to the back of a boat, hitting the gas, then flipping over when the rope tugs...
Inverted controls definitely more troublesome for some people, and I understand that. I'm really only justifying it because when I was a kid, I didn't know how to change the controls from inverted, so that's what I'm used to now XD
basically, when (or if) you play games, you know those two little analog sticks on your controller? The left one is usually looking around (press up you look up, down is down, etc...) and the one on the right is moving (go left, right etc...). Some people prefer "inverted" axis, and what that means is when they press the left analog stick up, they look down, or if they press the right analog stick up, they move backwards.
It used to be default/default, but one of my AC games got stuck on default x/inverted y for a while and by the time I thought about changing it, I was used to it. Now I just play all my games like that :3