After Ash accidentally destroys Mewtwo's cloning machine and frees Pikachu, Bulbasaur, Squirtle, Charizard and the other Pokémon. The first thing Pikachu, Bulbasaur and Squirtle do when freed is give Ash a hug.
The moment when Mew swoops in out of nowhere to save Ash just before Mewtwo nearly killed him. He didn't know Ash very well by that point, and he didn't have to save his life. But he did it.
The dub Mew gives a good speech about how wraith doesn't get you anywhere.
Mew: (Meowth translates) You don't prove anything by showing off a lot of special powers, a pokemon's real strength comes from the heart.
The scene where Ash gets turned to stone, and poor Pikachu keeps trying to wake him up started out very sad. It became a Moment Of Awesome when Pikachu's tears and not its electricity brought him back.
"I see now that the Circumstances of one's birth are irrelevant. It is what you do with the gift of life that determines who you are."
Even more so in the Japanese version, where Mewtwo's Heel Realization is connected to the plot/message that's been discussed and explored throughout the movie. He finally realizes that it's pointless to prove that his race of clones is superior through force, loses the ego that he's been trying to feed throughout the movie, and decides that life is just worth living to the fullest, coexisting peacefully with other humans and Pokémon.
The last lines of the movie make a great Bookend to the series in general. After Ash catches a glimpse of Mew he reminisces about his first steps.
Ash: Hey what's that?
Misty: What's what? I don't see anything.
Brock: Except the sky.
Ash: The day I left home to start my journey, I saw a really rare pokemon. And just now I thought I saw another one.