Right shoulder button. It feels natural and I got into the habit when playing the Street Pass Plaza Mini Games to move things along faster.
"L" button? Or "L" shoulder? I guess "L" shoulder. I only play New Leaf on my DS and have no experience with other games yet. Yeah, I know...what a one-game geek!
I usually use B because I have no patience whatsoever, but a lot of the time I end up mindlessly pressing B and then canceling something by accident (especially annoying if I'm talking to Cyrus ooohhh godddd so much dialogue). So I've been trying to use A more often recently, at least for certain conversations >__>