Arch is a distribution for advanced users. Its official installer provides only a command line, requiring you to bootstrap a system yourself. It's by definition the most customizable, because nothing happens unless you do it. If you're a new user, this distribution is probably going to be unstable, because you're pretty much guaranteed to miss something.
Perhaps Arch's greatest strength is the amount of software available in the AUR (Arch User Repository). It has upwards of 58000 packages, only 10K less than Debian's 68000.
Arch is also notable for following a rolling release model, which means frequent minor updates instead of infrequent major updates.