Here's a list of theme songs that I think would suit Ash Ketchum and his Pokémon.
Ash Ketchum: "Wake Me Up" by Avicii.
Pikachu: "Paradise City" by Guns N' Roses.
Ash's Pokémon (Kanto)
Butterfree: "Cool Kids" by Echosmith.
Pidgeot: "See You Again" by Wiz Khalifa ft. Charlie Puth.
Bulbasaur: "Headstrong" by Trapt.
Charizard: "Switchback" by Celldweller.
Squirtle: "Shell Shocked" by Juicy J, Wiz Khalifa, and Ty Dolla $ign feat Kill the Noise and Madsonik.
Kingler: "The Anthem" by Good Charloette.
Primeape: "Where the Hood At?" by DMX.
Muk: "Tell Me I'm a Wreck" by Every Avenue.
Tauros: "Chasing the Sun" by The Wanted.
Lapras: "Crawling Back" by Discrepancies.
Snorlax: "Battle Scars" by Guy Sebastian ft. Lupe Fiasco.
Ash's Pokémon (Johto)
Heracross: "Troublemaker" by Olly Murs, Ft. Flo Rida.
Bayleef: "Everytime We Touch" by Cascada.
Quilava: "My Songs Know What You Did in the Dark (Light 'Em Up)" by Fall Out Boy.
Totodile: "My Life Is a Party" by ItaloBrothers.
Noctowl: "Sky Full of Stars" by Coldplay.
Donphan: "Push It to the Limit" by Paul Engemann.
Ash's Pokémon (Hoenn)
Swellow: "Danger Zone" by Kenny Loggins.
Sceptile: "Never Too Late" by Three Days Grace.
Corphish: "One More Night" by Maroon 5.
Torkoal: "I Cry" by Flo Rida.
Glalie: "Ice Ice Baby" by Vanilla Ice.
Ash's Pokémon (Sinnoh)
Staraptor: "Every Other Time" by LFO.
Torterra: "End of Me" by Ashes Remain.
Infernape: "Invincible" by Skillet.
Buizel: "Ridin' Solo" by Jason Derulo.
Gliscor: "What's my Age Again?" by blink-182.
Gible: "Dare to be Stupid" by "Weird Al" Yankovic.
Ash's Pokémon (Unova)
Unfezant: "Around the World" by ATC/A Touch of Class.
Oshawott: "Hey Juliet" by LMNT.
Pignite: "Fire Burning" by Sean Kingston.
Snivy: "Live for the Night" by Krewella.
Scraggy: "I'm Just a Kid" by Simple Plan.
Leavanny: "Sunflower" by Post Malone.
Palpitoad: "96 Quite Bitter Beings" by CKY.
Boldore: "Sandstorm" by Darude.
Krookodile: "To Be Loved" by Papa Roach.
Ash's Pokémon (Kalos)
Greninja: "What I'm Made Of" by Crush 40.
Talonflame: "Courtesy Call" by Thousand Foot Krutch.
Hawlucha: "Livin' La Vida Loca" by Ricky Martin.
Goodra: "Follow You" by Imagine Dragons.
Noivern: "World on Fire" by Daughtry.
Ash's Pokémon (Alola)
Rowlet: "Lazy Days (Livin' in Paradise)" by Ted Poley.
Lycanroc: "Animal I Have Become" by Three Days Grace.
Incineroar: "Let the Bad Times Roll" by The Offspring.
Naganadel: "Grenade" by Bruno Mars.
Melmetal: "Jump" by Kriss Kross.
Ash's Pokémon (Galar)
Dragonite: "High Hopes" by Panic! At the Disco.
Gengar: "Ghost Town" by Shiny Toy Guns.
Lucario: "Hall of Fame" by The Script.
Sirfetch'd: "This Is It" by Oh the Larceny.
Dracovish: "Walk the Dinosaur" by Was (Not Was).
Honorable Mention(s)
"California Love" by 2Pac ft. Dr. Dre.
"Dear Maria, Count Me In" by All Time Low.
"Smell Like Teen Spirit" by Nirvana.
"Princes of the Universe" by Queen.
"All Downhill from Here" by Newfound Glory.
"Lonely Day" by System of a Down.
"Rawkfist" by Thousand Foot Krutch.
