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.
At first, Android 17 and 18 were much stronger than Piccolo. Then in Chapter 360, he fused with Kami which made him much stronger. At this point, he was already strong enough to take on Android 17 one on one. By the end of the Cell Saga, Piccolo has trained in the Hyperbolic Time Chamber and become much stronger than the Androids.
Since then, Piccolo has never slacked off in his training and it is therefore safe to say that he is much stronger than the Androids. Since Gohan has not been training all this time and has become a lot weaker, therefore at this point Piccolo is without contest the 4th strongest of the Z fighters (after Goku, Vegeta and Buu - not counting Gotenks).
Actually because 18 is still part human she has the ability to get stronger with training, that's already a known fact, android 16 is the one who is stuck, and actually gohan was confirmed to be alit stronger then buu because buu is weaker than he would be if he was still with kid buu, also it was said that android 18 is stronger than all the guys fighting frieza and his men, this includes piccolo, not including goku and vegeta who weren't there at the time,so yeah check your facts guys lol, 18 said it herself and krillin agreed lol but because piccolo was training for 6 months before the tournament his power now surpasses 18s so at the time goku chose piccolo he was stronger than 18 but since buu is weaker than gohan and there was a written test I think 18 would've been a better choice than buu
In Dragon Ball Z, after fusing with Kamisama, Piccolo got as strong as 17, which supposedly is slightly more powerful than 18. After that, he went to time chamber and got even stronger. Then, by the time of the fight with Super Cell, Piccolo was already stronger than 18. Considering Piccolo is a fighter who is always training and increasing his power level, he should be considerably stronger than 18 by the time of Dragon Ball Super