Debian is very well suited for application servers, web servers, file servers, hypervisors (personally, we use Proxmox for some hypervisors and this is based on Debian), and much more. It can run on various CPU architectures and scales well from large to small. We have Debian DNS servers running on Raspberry Pis, and large application and database servers running on powerful Debian installs in our racks.
If you want an OS that is rock solid, easy to manage, and predictable, Debian might be the OS you're looking for. The official packages are generally far from being bleeding edge versions, but this means that you are using packages that are battle tested and will not break compatibility at any point during its life-cycle. This, to me, is one of Debian's core strengths and a good reason for recommending it to others.
On the flip-side, the lack of up-to-date versions of various packages may be an annoyance if you actually do need to be using the latest version of such a package. If you need to be using more modern versions, you will need to remedy that yourself or look at an alternative distribution. Ubuntu may be worth a look, in this case, being that it is basically Debian with newer packages and some minor differences here and there.
Personally, I would not recommend Debian for a graphical desktop OS. You'll nearly always be better off with one of the Ubuntu flavors for a modern desktop with all the bells and whistles you'd expect with a GUI desktop.
SLES is a good fit for running supported enterprise applications like SAP, HANA, oracle,.. We use it to run all our SAP workloads, and so far everything has worked great. All libraries are right in the repository, all documentation is correct and the support answers really fast in case you have problems.
The support window for service packs after a new SP is released is too short.
Community engagement is low.
There are times when supported packages fall too far behind and create compatibility issues with applications. The Open Build Service usually provides a way around this, though.
Debian is one of the most well-thought-out, logical, and intuitive Linux operating systems you can use. Configuration, package management, file system layouts, etc, are all based on many years of usage, and it's very rare to find anything surprising or confusing in everyday usage. Package management and updates are handled by APT in such a way that once configured, you can almost guarantee your system will behave as expected for the lifetime of its service. Top-notch system integration provides all the tools you'd expect from a modern Linux system, with all the stability you demand from a demanding server OS.
For the breadth of services, features and overall performance, I believe Suse Linux is a great choice for any enterprise. It still has to grow a bit in areas like online help forums and documents, but we are pretty much satisfied with our choice.
It's great for basic support issues. But it's less suited for anything that is either obscure or uncommon, and it is very hard to escalate, unless you've built a relationship with your vendor/salesperson.
SUSE Linux Enterprise Server works well with the SUSE Manager, it really simplifies everything. We also do have SUSE 4 SAP, and the SUSE Manager is able to manage them all, even other (up to date) Linux distributions. Patching has never been that easy and the SALT integration is also working without any problems.
It allows a very small team to manage a large number of servers, due to its automation and manageability.
SAP supports it very well, making it straightforward to manage. There are also "recipes" for Oracle which has all of the requisites to install an Oracle DB; which helps with deployment immensely.
It lets us run large SAP instances in SLES on Power, helping us with support, manageability, and performance.
Support costs are high, but required for SLES for SAP.
