Honestly, since you're new, don't just take our advice. There are plenty of links in the download section where you can sample at your leisure. As many pointed out, Ayumi is
very versatile in her music. Some people may not like her older works for some reason and some might not like her newer works for some reason, but every one has something unique about it, and honestly what some one else might not like could be something you can really jam to.
My advice check
here
and do what I did: go "eeney meeney miney moe" select a few tracks from each album. If you really like some, keep going until you make a decision, on what album(s) would be the best one for you to invest your money in. And honestly, you don't have to buy the entire discography to be a fan; the music isn't the fancy album cover.
best of luck.