I would choose James' passage on the effectiveness of prayer:
13 Is anyone among you in trouble? Let them pray. Is anyone happy? Let them sing songs of praise. 14 Is anyone among you sick? Let them call the elders of the church to pray over them and anoint them with oil in the name of the Lord. 15 And the prayer offered in faith will make the sick person well; the Lord will raise them up. If they have sinned, they will be forgiven. 16 Therefore confess your sins to each other and pray for each other so that you may be healed. The prayer of a righteous person is powerful and effective. (James 5:13-16)I have heard and read many discussions of this passage from admired pastors and respected theologians. None of them satisfy. The plain reading of the text is that prayer can make the sick well. This is, quite frankly, neither demonstrable or logical. Not demonstrable because we pray for sick people all the time--and they sometimes recover and sometimes not, at normal rates. Not logical because it is our destiny to have our three score and ten and then, to first order, get sick and die.
No, I wouldn't choose having creation, or the end-times, or infant vs. believer's baptism, or the problem of evil, or anything else-- given one opportunity to have my eyes opened on one item--I would use it on the question of prayer. This passage in James bothers me so much I confess I sometimes wonder if James is really canonical. I will continue to go on the assumption it is, given I know that for the most part I'm an idiot. But I will probably never understand its straightforward teaching on the tangible effectiveness of prayer.