For ESRI its almost always going to be:
Lat = Y Long = X
It's easy to get backwards. I've been doing this for years but still need to think about it sometimes.
On a standard north facing map, latitude is represented by horizontal lines, which go up and down (North and South) the Y axis. Its easy to think that since they are horizontal lines, they would be on the x axis, but they are not.
So similarly, the X axis is Longitude, as the values shift left to right (East and West) along the X axis. Confusing for the same reason since on a north facing map, these lines are vertical.
I'm mildly dyslexic so I always need to pause and think about it for a brief second when displaying new x/y data. Hope this helps.