Why didn't Goku choose Android 18 for the tournament instead of Piccolo? In Dragon Ball Super Manga Chapter 6, Goku chose Piccolo for Beerus' team. But why didn't he choose Android 18? She was far more powerful than Piccolo in Dragon Ball Z. Is it because her power level decreased just like Gohan?
There are many reasons to the answer of this question:


The primary being that as of now, due to having kept up his training, Piccolo is far stronger than Android 18. Also he has more potential than the Androids. They were created by Dr. Gero with the power level they have, whereas Piccolo became this strong due to his training and fusion with Kami, and Nail.
Another reason is that since Goku is of the caring nature, and knowing the training procedure and the time spent, would not want to interfere with the upbringing of Marron. 
It can also be said that since Piccolo is Akira's favorite character (read here), he wouldn't want to leave him behind, as a weakling, when his least favorite character, Vegeta (read here), gets such a power boost as SSB.
He doesn't have as much potential as Gohan, who was recommended by Vegeta to be selected into Beerus' team, but Goku said that he's a scholar now, so let him be. And Gotenks, being stronger than Piccolo, has a time limit. 


So it's safe to say that Piccolo is the most appropriate person to be on Beerus' team.