I'd have to say the most amazing, heart wrenching miracles I've witnessed, although I've seen major debilitating diseases healed, is the power of the Holy Spirit overcoming someone in the pits of despair (so called castaways of society, drunks, homeless, depressed, suicidal, or just average folks etc..). When you're preaching the actual gospel of reconciliation, the death, burial and resurrection of Christ (the power of God to salvation) and you pray someone through repentance and acceptance of Christ as Lord and see them transformed right before your eyes by the Holy Spirit, never to return to a debauched, hopeless lifestyle, there are no words to describe that. That's the true power of the Gospel unadulterated.
As Chalam pointed out:
Romans 8:2 (New International Version)
2 because through Christ Jesus the law of the Spirit of life set me free from the law of sin and death.
I pray that some day all JW's will get to witness the real working of the Holy Spirit.