So, out of all the ways of practicing devotion, if you just remember smaran (remembrance), that is the main thing. Smaran is the pran (life). Smaran is the essence and the life of devotion.
If we leave the life (of devotion) out, then what is the use of everything else? Although three are given prime importance - shravan, kirtan, smaran - but kirtan without smaran and shravan without smaran have no real devotional value, although they are good helpers.
Your mind has been actively engaged in thinking about Shri Krishna for the last hour. For the first half hour you were doing kirtan and your mind was in Krishna. And for the last half hour you've been doing shravan and your mind has been in Krishna, and for most people that's easier than if I tell you to sit silently and only do smaran.
Your mind wouldn't have stayed in Krishna as much in the last hour as it did when you were doing kirtan and shravan. So the kirtan and shravan are important parts of devotion as long as we remember that they are tools. They are to help us engage our mind in remembering Shri Krishna.
But the essence of devotion is to attach our mind to Him through this smaran. By remembering Him alone does our mind get purified.
