What are the most effective messages to preach in a youth service?

The Gospel. While this may seem like a overly simplified answer, the greatest mistakes that youth pastors tend to make is to try and make every message conform to what they think the youth want to hear. Young people today have a very strong desire for "real" leadership and can see quickly through all of the fluff that pastors are trying to feed them. Trust your young congregation with full and unabridged lessons from scripture and you will find that real leaders with real hunger for righteousness will come out of your congregation. When it comes to being effective and relatable, tie this real Biblical teaching to ways that they can apply it at home and in school.