There are a lot of reasons to choose Debian as your operating system – as a user, as a developer, and even in enterprise environments. Most users appreciate the stability, and the smooth upgrade processes of both packages and the entire distribution. Debian is also widely used by software and hardware developers because it runs on numerous architectures and devices, offers a public bug tracker and other tools for developers. If you plan to use Debian in a professional environment, there are additional benefits like LTS versions and cloud images.

For me it's the perfect level of ease of use and stability. I've used various different distributions over the years but Debian is the only one that just works. NorhamsFinest on Reddit
Debian for Users

Debian is Free Software.
    Debian is made of free and open source software and will always be 100% free. Free for anyone to use, modify, and distribute. This is our main promise to our users. It's also free of cost. 

Debian is stable and secure.
    Debian is a Linux-based operating system for a wide range of devices including laptops, desktops and servers. We provide a reasonable default configuration for every package as well as regular security updates during the packages' lifetimes. 

Debian has extensive Hardware Support.
    Most hardware is supported by the Linux kernel which means that Debian will support it as well. Proprietary drivers for hardware are available if necessary. 

Debian offers a flexible Installer.
    Our Live CD is for everyone who wants to give Debian a try before installing it. It also includes the Calamares installer which makes it easy to install Debian from the live system. More experienced users can use the Debian installer with more options for fine-tuning, including the possibility to use an automated network installation tool. 

Debian provides smooth Upgrades.
    It's easy to keep our operating system up-to-date, whether you want to upgrade to a completely new release or just update a single package. 

Debian is the Base for many other Distributions.
    Many popular Linux distributions, like Ubuntu, Knoppix, PureOS or Tails, are based on Debian. We provide all the tools so that everyone can extend the software packages from the Debian archive with their own packages if needed. 

The Debian Project is a Community.
    Everyone can be a part of our community; you don't have to be a developer or sysadmin. Debian has a democratic governance structure. Since all members of the Debian project have equal rights, Debian cannot be controlled by a single company. Our developers are from more than 60 different countries, and Debian itself is translated into more than 80 languages. 

The reason behind Debian’s status as a developer’s operating system is the large number of packages and software support, which are important for developers. It’s highly recommended for advanced programmers and system administrators. Adarsh Verma on Fossbytes
Debian for Developers

Multiple Hardware Architectures
    Debian supports a long list of CPU architectures, including amd64, i386, multiple versions of ARM and MIPS, POWER7, POWER8, IBM System z and RISC-V. Debian is also available for niche architectures. 

IoT and Embedded Devices
    Debian runs on a wide range of devices, like the Raspberry Pi, variants of QNAP, mobile devices, home routers and a lot of Single Board Computers (SBC). 

Huge Number of Software Packages
    Debian has a large number of packages (currently in stable: 59000 packages) which use the deb format. 

Different Releases
    Besides our stable release, you can install newer software versions by using the testing or unstable releases. 

Public Bug Tracker
    Our Debian bug tracking system (BTS) is publicly available for everybody via a web browser. We do not hide our software bugs, and you can easily submit new bug reports or join the discussion. 

Debian Policy and Developer Tools
    Debian offers high-quality software. To learn more about our standards, read the policy which defines technical requirements for every package included in the distribution. Our Continuous Integration strategy involves Autopkgtest (runs tests on packages), Piuparts (tests installation, upgrade and removal), and Lintian (checks packages for inconsistencies and errors). 

Stability is synonym with Debian. [...] Security is one of the most important Debian features. Christos Pontikis on pontikis.net
Debian for Enterprise Environments

Debian is reliable.
    Debian proves its reliability every day in thousands of real world scenarios, ranging from single user laptops to super-colliders, stock exchanges and the automotive industry. It's also popular in the academic world, in science and in the public sector. 

Debian has many Experts.
    Our package maintainers do not only take care of the Debian packaging and incorporating new upstream versions. Often they're experts on the application itself and therefore contribute to upstream development directly. 

Debian is secure.
    Debian offers security support for its stable releases. Many other distributions and security researchers rely on Debian's security tracker. 

Long Term Support
    Debian's free of charge Long Term Support (LTS) version extends the lifetime of all Debian stable releases to at least 5 years. Additionally, the commercial Extended LTS initiative supports a limited set of packages for more than 5 years. 

Cloud Images
    Official cloud images are available for all major cloud platforms. We also provide the tools and configuration so you can build your own customized cloud images. You can also use Debian in virtual machines on the desktop or in a container