Well buying my first package of adult diapers was a major and traumatic decision. But putting my first on and wetting it was so conflicting, but exciting, I did not realize the unconscious decision I made to wet my diaper standing in front of the mirror my first time until after I did it. Then to realize this may have been at age 42 the first time I ever peed standing fully erect.
Now I wear diapers mostly full time and I wet my diaper by choice standing most of the time, so by default I have to say my favorite diaper wetting position is standing.
I also smoke cigarettes (I know, but please don't say anything negative, I really need them) and I time most of my diaper wetting with when I need a cigarette. I love standing on my deck, nothing on but my adult diaper, inhaling my cigarette smoke while slowly wetting my diaper. Sublime.