For me it was Clocks, I was about 7 or 8 years old, and I remember that my dad was obsessed by that song... (and also now he's obsessed!)
I think that now is one of the songs that I like most... thanks to my dad!
Yellow and then what really got me hooked was seeing them perform on tv. I loved how they looked like they were absolutely in love with their own music and seemed to just lose themselves in the music. Watching Chris play in clocks really got me hook line and sinker though.