If you're gonna buy ONE album I'd go for Duty, that's an allrounder and it's so great. I actually don't have it myself
YET but I really want it. I am... is a great choice too. You should prolly save A Song For XX to buy last, it's the least liked album and it's not really Ayu like many ppl like/know her. It's got great tracks tho, including one of my favs (Powder Snow).