What OS do you prefer for servers, Windows or Linux? Why?

Linux..

Compared to Windows Server, Linux handles more functions without issue, and doesn’t require reboots nearly as often. That’s because Linux doesn’t have memory leaks in the same fashion as Windows Server, and only needs to reboot when there’s a kernel update

I prefer Windows because I work with the Microsoft Blazor framework and also have complete compatibility with other Microsoft products like Azure, DevOps, SQL Server, and so on.

Both Operating Systems have their own pros and cons. If we talk about windows, it has user friendly interface but needs to be upgradation. On the flip side, Linux is an open source and more secure Operating System. 

Linux is made for servers and Windows been made for desktop.

Windows server license is very high compared to Linux almost free of charge. 

Linux also more rubost and secure. Updates on Windows servers takes ages and faulty to complete 99% of the time. 

Most of the web apps that gets hacked are hosted on Windows servers. 

LINUX for his security and modularity

it depends to users nessassary.we can compare them for example linux is open source but windows no.windows support asp and asp.net but linux no because it is microsoft product.linux supprt php,python,perl,cgi but somtimes php installed on windows has some problem.windows support SQL Serer but Linux support My Sql.so it depends ...

Linux.
considering the cost, customizability and security Linux would be prefferd over windows.

Linux is open-source, cost-effective, secure, and stable.

First tell me what you want to do? If you intend to use Microsoft products or .NET programming language and SQL database, then I recommend choosing Windows Server. However, for open-source programming languages, web hosting, load balancing, and other such applications, Linux would be the ideal choice. Of course, the choice ultimately depends on the experience and expertise of the team. While Windows is easy and visual, Linux may require more expertise.

Linux

Cause it's so popular for servers so it leads to have much bigger community and so much more information and tools

It depends, servers are mere hardware, you acquire them to run certain applications.

Let say that I say Linux preferably (Red Hat) is prefered OS for servers, which is also correct to certain extent too,

Lets take an example to understand this, the person who has posted the question is working with "Medical and Diagnostic Center" and assume that his "Medical and Diagnostic Center" is using certain windows or Mac based software to run its operations, then it won't matter whether Linux is good or not. Because for the "Medical and Diagnostic Center" in this example will require windows server any way.

In general context usually Linux distribution are open source, have better security implementations, and are generally efficient.