Expelliarmus is obviously Harry’s spell. It has saved him many times, and he defended it harshly when Zacharias Smith mocked his affection for it. To me, the spell symbolises Harry’s disinclination to cause anyone pain. He would far prefer to disarm someone than to use a spell that could cause them serious damage, even when fighting against you know who during the battle of Hogwarts. The spell also reflects Harry’s past. When we first meet Harry, he is a poor boy [from a poor family! Sorry, couldn’t resist.] who is teased and basically not really considered anything particularly impressive. But look at what he does! The disarming spell is the same: It is considered “nothing special” but is ultimately the spell that defeats Tom Riddle.
“Harry, the time for Disarming is past! These people are trying to capture and kill you! At least Stun if you aren’t prepared to kill!” “We were hundreds of feet up! Stan’s not himself, and if I Stunned him and he’d fallen, he’d have died the same as if I’d used Avada Kedavra! Expelliarmus saved me from Voldemort two years ago.”
