My favourite is Satoshi/Ash.
Hold on. I'll tell you why.
Putting the matter of him never ageing aside, Ash has been through a lot. He's journeyed through five regions and two archipelagos, competed in many tournaments, and met dozens of legendary Pokemon.
He's also had to say goodbye to a lot of friends. Charizard, Squirtle, Lapras, Butterfree, the list goes on. Not to mention the humans he's had to leave behind, like Misty and Brock. The one friend that's stayed by his side all this time is Pikachu. Pikachu is the one being in the world (although Charizard comes a close second) who understands what Ash has been through, and that sense of loneliness can weigh on a person. Which is why sometimes I like to imagine, when Iris calls Ash a little kid, or Cilan acts like a faggoty replacement for Brock, he thinks to himself that he doesn't need to take this shit. He's left better people for worse reasons, and Iris and Cilan should count themselves lucky that they get to travel with the trainer who fought Mewtwo, who braved the waters in the Lugia movie.
Sometimes Ash and Pikachu lie awake thinking of their experiences, and wonder why they spend their time with such colossally useless people as Iris or Cilan.
tl;dr pretend that Ash ages mentally and emotionally even if he doesn't physically, and he's seen some shit.
