I can't believe Pikachu has won 15 battles. People always complain that he loses a lot, but he has more victories than any of Ash's other Pokemon. Color me impressed.
It's a bit of an illusion. We see Pikachu battle
a lot more then the other Pokemon. We kind of get used to seeing him winning against extremely powerful opponents, and when he loses people get surprised because he's so experienced. You don't really think of it from a " Pikachu is a living breathing thing in the anime that makes mistakes " angle more then a " Pikachu is so strong I can't believe it lost to a Snivy " angle. His losses seem to mean more to people because there's less to justify them beyond anime logic.
Since people seem interested I'll do the battling strength for the main Pokemon, too:
[spoil]
Ash:
Pikachu: 75%
Unfezant: 50%
Pignite: 47%
Oshawott: 47%
Snivy: 55%
Scraggy: 42%
Levanny: 67%
Palpitoad: 50%
Boldore: 67%
Krokorok: 38%
Iris:
Axew: 33%
Exacdrill: 50% (In Series. 96% including Pre-Series wins.)
Emolga: 100%
Cilian:
Pansage: 50%
Crustle: 89%
Stunfish: 0%
Top 3: Emolga, Pikachu, Crustle
Bottom 3: Stunfish, Axew, Krokorok [/spoil]
Rivals!
Bianca: 33% (On Screen. 45% otherwise.)
Trip: 54%
Burgundy: 0%
Georgia: 57%
Stephan: 71%
Interesting that Ash and Trip are even now.