"Ach!" is one of the most flexible words ever invented. I have to admit I thoroughly love it. It has almost as many different meanings as "Fuck!" has in British English and shares many of them. Depending on the emphasis and tone, it can convey every emotion known to mankind, from frustration to ecstasy and beyond.
To my great surprise, when the most beautiful German word was elected in late 2004 (yes, we Germans do such things), "Ach" wasn't even mentioned. The shallow and comfortable "Habseligkeiten" (meaning belongings) was voted as number one. And that despite the fact that "Ach" also forms the ending of Johan Sebastian bACH, as in this case ;-)