When I discovered Ayu in 2006, the only song of hers I really liked was Angel's Song
... but I liked it so much I was willing to stick with her even though I didn't really like any of her other songs at the time. Originally, I dismissed M and Free & Easy because I had only heard remixes; but then I heard the originals and got to appreciate the genius behind them. M, right away; Free & Easy took a while to grow on me though.
This all happened around 2007/2008 though. Right now, the lists of songs I like and dislike is pretty stable.