I'm going to have to go with Only One by Yellowcard
Honestly, I could go with Ocean Avenue but this song is my favorite by Yellowcard. The drums are absolutely killer(Longineu is a very underrated drummer, also didn't know that he was French) and I love the Violins in the bridge(I hope they are Violins). This is a song that I loved on the first listen years ago, and a song that is very deserving of this day.
~Honorable Mentions~
Check Yes Juliet by We The Kings:This is very nostalgic for me and I listen to it often to this very day. I am not a huge "We The Kings" fan but this was a very good song and it's something in my current playlist(not sure if I will ever remove it).
Halfway Gone by Lifehouse:Honestly, Lifehouse is criminally underrated(I know underrated is a term that is overused but it works here). I can name off the top of my head almost 10 songs I like by them yet I never listen to them anymore. This song is the obvious exception, and I really love the music video. It's simplistic and although it doesn't have a story to it, it still looks like it would've been really fun to shoot.