Just so you know where I'm coming from, I have led Bible studies and have been a university Theology professor whose doctoral specialization was New Testament, Judaism, and Greco-Roman backgrounds. I think the more interesting question is this: If you had to choose, would you participate in a group Bible study or a small prayer group? For me, the answer is a no brainer: the small prayer group. I am now a retired UMC pastor, but I continue to participate in our little interdenominational prayer group with 5 or 6 regulars. We meet for 2 hours every Monday and we have seen many healing miracles and other marvelous answers to prayer. So the choice in question is no contest. Small prayer groups have triggered the greatest historic revivals: the Great Awakening, the Second Great Awakening, the Welsh Revival, the Azusa Street Revival, and, lastly, the great Hebrides Revival. In those revivals, the Holy Spirit fell not primarily on believers, but on unchurched sinners and thousands were saved in a short period. Whole countries were "soaked" in God's presence and the great masses sensed this and were overwhelmed by its convicting power.
Private prayer, pastor-led prayer in church, and prayer chains are not even remotely as effective as a weekly small group that meets primarily for prayer. In the corporate body of Christ, the collective is far more spiritually effective as a group acting in harmony than its individual members. People attend church for many reasons besides prayer and prayer chains are conducive to mechanical laundry list type prayers. But the very act of driving to a special location for group prayer once a week is already an act of faith!
As for the most powerful form of individual prayer, Paul singles out "praying in the Spirit" as the key to waging spiritual warfare (Ephesians 6:11-18, especially vs. 18). Praying in the Spirit is the hardest, most demanding, and yet, the most rewarding form of individual prayer. It is far, far harder than Bible devotions.
Your thread has inspired me to start 2 new threads to demonstrate what I'm saying here: one thread on how prayer triggered the Great Historic Revivals and another on how to "pray in the Spirit" (and no. I'm not primarily talking about speaking in tongues here..
Watch the documentary posted at the beginning of my Speaking in Tongues thread and you'll see how a small prayer group of poor Blacks started a movement that has led to 600 million converts, i. e. a quarter of the world's Christians!
I'm convinced that the main reason we haven't had a great revival since the Hebrides Revival of 1949-1954 is neither because of sinfulness in society and the church or because of divine sovereignty, but because believers are too spiritually slothful to study and emulate the prayerful quests of spiritual giants of the past whose prayer life brought some of the mightiest awakenings the world has ever seen! The spiritual condition of America was just as grim prior to the Great Awakening as it is today!