This depends. Servers range in price from a few hundred dollars for a NUC up to $35K for a Taiko. If you are at the very high end, you want to take into account what is your preferred connection, what clock you are going to use, etc... Do you have a renderer or roon endpoint? Which one?
It is always optimal to have server and renderer on different devices when possible. This is also where a bit more processing power is desirable since this is what is interfacing with the internet.
Taiko is undeniably the best, but $35K. There are many degrees between NUC and Taiko. A few options:
Roon has their own servers, the Nucleus and Nucleus+. These are lovely, very quiet devices that are obviously designed around running roon core. They will also serve as a rendered. The difference in need between a Nucleus and Nucleus+ is driven by size of your library and your DSP. These are very good options for devices that are under $2K nd $3K respectively.
Antipodes - they offer multiple server solutions as the S30, S40 and K40 that steadily get better. The cool thing about Antipodes is that they allow for upgrades. You can use add an external power supply, clock and ripper. These range from $3K to $9K. The K40 is extremely appealing as its goal is to be a brilliant server with the best ethernet connection to your renderer possible. For a straight server, the K40 is insanely good at $9K. The K50 is their ultimate device but is streamer, player and clock in one box.
There are a variety of other options including Innuous, Rockna, Auralic, Aurender, etc.... Rockna is insanely good but best with their DACs.
I am a Roon, Antipodes and Rockna dealer, I have no association with any other brands mentioned.