The key assignment for me changes with each generation.
Gen 2- It was usually the bike, and when near a stream, a Rod of some sort
Gen 3- In my first playthrough, I registered the Item Finder, since for me,the running shoes' speed was competent. with FR/LG though, I almost always registered the VS Seeker.
Gen 4- last I played I spent most of my time in the underground attempting to get rare items. So Explorer Kit wins for now.