I've only been a fan of ayu's since 2007...so it's kinda hard for me to recall my favorite songs from the years before that, so I'll just go with my favorites since '07.
2007: talkin' 2 myself
2008: GUILTY
2009: Sparkle
2010: Microphone (but "Like A Doll" is a close second
)
2011: BRILLANTE
2012: NaNaNa
2013: Merry-go-round
2014: XOXO
2015: Sayonara