Prayer from the heart, prayer emanating from true belief, these are the most powerful prayers. James 5:16 says the prayers of righteous people are automatically powerful and effective,
One can pray the Rosary or the Psalms by rote, without taking them into the heart and thus not allowing them to emanate from true belief. Either one is less powerful than any other more devout prayer when done this way.
The saints have expressed many differing opinions about what is the most powerful prayer.
A sincere and devout Sign of the Cross can be a very effective prayer against evil.
Peter Kreeft wrote: I am now going to tell you about the shortest, simplest, and most powerful prayer in the world. It is called the “Jesus Prayer”, and it consists simply in uttering the single word “Jesus” (or “Lord Jesus”, or “Lord Jesus Christ, have mercy on me, a sinner”) in any situation, at any time and place, either aloud or silently.
Many have said that the Mass is the most powerful, most perfect prayer.