what is the best computer for freelance graphic designer?

You might start here:

Mac - Which Mac is best for me? - Apple

You should also talk to other people you know who use the same or similar software.

+1 on chatting up your peers. There may be apps you'll need that are available for Windows but not for macOS.

The iMac Pro with a very nice 27-inch monitor was discontinued almost 5 years ago. Its powerful Intel processors wit not run some features of newer macOS versions. Although a great computer, it is, IMHO, now too old to consider picking up on the used market.

We might do a better job of suggesting if we knew a little more:

— What apps do you need to run?

— How many monitors do you see using and what specs?

— Do you have workspace limitations?

— How much storage do you need now? In two years?

That may help us better help you. Cheers!

AJ