Stable, secure, it just works, has one of the, if not THE largest software package repo of all Linux distros, has lots of third party support for proprietary software and drivers that are available as .deb files or through official PPAs. It also is not backed by any corporation, but is a community developed distro. You can install it on pretty much everything.

The only downside I would say is their shift into using Systemd. They shouldn't have done that. It was forced through undemocratically and I think that was a big mistake, even though they are trying to limit software dependencies to it. There's a fork called Devuan that I've been considering where you can pick your init system. (SysVinit/runit, etc) I honestly miss SysVinit. It was simple, easy to understand and easy to maintain.