Which computer do recommended

The platform could be either Mac or Windows. Macs have not dominated graphic design for decades, especially when including corporate design departments. It does tend to be preferred among freelance designers, however. (I own and use both platforms equally well. Lest anything I don't like Macs, I have been a Mac user since 1985, and an Apple Authorized Consultant from 1990-2011.) 

You need to ask your self a couple of questions...

Desktop or laptop? 

What is your budget?

Find the system you like and spec it out to the best the company allows. Then cut back to meet your budget. (Obviously, you will go to Apple for Macs; I prefer Dell for my laptops; I like to build my own Windows towers.)

Which field of graphic design are you considering? There are lots of niche fields that use a lot of graphics work (print publishing, print advertising, digital illustration, typeface design, point-of-purchase advertising, package design, motion graphics for video, signs/environmental design, outdoor advertising, game design, user interface design, etc). Some of these fields need more powerful systems than others. A computer system that's intended to do video production or 3D modeling/rendering work will probably cost more than a computer that's only going to be used for print page layout.

Like Bill said, you'll probably be playing it the most safe by buying an Apple Mac-based system. The Mac platform is dominant in more of the design fields I mentioned than Windows-based PCs. But there are some niche fields where companies are using mostly or only Windows-based PCs due to industry specific software and hardware in use (the sign industry and outdoor advertising industry are examples). I would still recommend getting a Mac if you don't know for sure where you could end up working -or just working for yourself as a freelancer. 

The other advantage of buying a Mac is Apple is the only company making them. They're usually going to be very dependable and have very good build quality. There are some very good Windows-based PCs out there and a lot of PCs that are not so good. You generally have to do a lot more research on various Windows-based systems before buying. It's especially tricky when considering a notebook computer purchase.

If you are already an experienced computer user you can probably ignore this next part. Lots of computer users, even many doing graphic design work, don't really know basic tasks like properly managing files. Typically they'll memorize a series of steps how to complete certain tasks in the applications they use, but the overall computer system remains a mystery. I think it's pretty easy for a person to use a Mac or a Windows PC if they're already adept at using the computer's file system. The Finder in a Mac or File Explorer in a PC are both visual-based tools that are easy to use. It's not like going back in time to the early 1980's where computers had no graphical user interface and ran off commands typed into a command line on a black monochrome screen. Some computer users these days just don't know how good they have it. Anyway, the point of all that is a person who is really computer literate isn't going to have a problem finding his/her way around on a Mac or PC. That should be the metric employers use when looking at potential employees rather than just seeing if they say they "know" Mac or PC on their resume.

In the professional world most graphic design has been traditionally done on Macintosh. I personally have found Macs excellent for designing and illustrating. If you are planning to work professionally for a company its almost certain that they will be doing design on Macs. The Creative Cloud programs are available for PC but since there are so many companies manufacturing PCs it may be more difficult to find one with the speed and appropriate video card that it is necessary for doing graphics.