For Ash you could probably consider any of the theme songs Rica sings because they're mostly sung from Ash's POV. I just listed Mezase Pokemon Master because it was the first one.
Which character had the best image (theme) song in your opinion? You can also count the music cues that play when a character does something (like Team Rocket's scheming music or Cilan's "It's ___ time!" music." My favorite is Maemuki Rocket Dan mainly because I just love Team Rocket, and the actual ending is so funny with them dancing. Takeshi's Paradise gets a mention for the same reason. I laugh everytime I hear that song. I like May's image song (Watashi Makenai) but she should have gotten an image song earlier. They waited until her last few episodes to give her a song, and when it was first used in the anime it was just a random image song. We didn't know it would actually be her image song until a few episodes later. They did Dawn's theme right by showing it early on and having it play during all of her contests and even in a few movies. I don't think all of those remixes of it was necessary though, and she should have sung it from the beginning.