Debian is one of the original distros, dating back to 1993. The Debian package management system is generally considered superior to the RPM systems, although they have closed the gap (mainly by incorporating features from Debian’s system). More here
Not only is there a large spread of software in Debian, but it is also available for eleven different CPU architectures. No wonder their release cycle is so long, that’s a lot of testing and quality control. One of Debian’s great advantages is that it is always possible to upgrade from one version to the next, without reinstalling and having to reconfigure everything from scratch.