There is no ONE thing. Learning scales and chords is great, but if you don't know how to apply them, what's the point? Learning songs is great, but if you can't take what you learned and apply it elsewhere, what's the point. Playing with other people is great, but if you don't know what to play, what's the point. We, as musicians, need to stop looking for the ONE thing and realize that there is no ONE thing. If you want to improve as a player - practice (by yourself), rehearse (with a band), and perform (for an audience). There is no other way; no shortcuts; no secrets; no tricks.