Throughout the entire series we've seen Ash put his own well-being aside to help out other people, trainers and Pokemon. Putting his journey on hold for whatever reason it maybe; finding a lost Pokemon, reuniting a parent and child, falling off cliffs and climbing mountains to save Pokemon, heck even helping to save the world on more than one occasion, whatever the case maybe.
What is your favorite act of heroism and selflessness that Ash has done since he left Pallet? I know there are too many to list so pick whichever ones stand out the most to you.
For me it will always be that moment when he stood up to the Spearow to protect Pikachu in episode 1. Here we see another side to the rookie trainer for a Pokemon that didn't even care for him and from then on they became best friends for life.
Another which is probably #2 or tied with my #1 would be in the first movie when he sacrificed himself to stop the fighting between Mewtwo and Mew.