Was asked about best OS for programming/designing, won't be talking about the best one to do that work in, that's subjective and there are so many OSes out there. Will speak from pure practicality, what you should know what to program in. You should know Windows programming. You know Windows programming, that gets you the PC market, and the similar Xbox market. Just learn PlayStation after that and you have most of the market. This presupposes you know some form of C language but Cain will just talk about OSes.
Started on Atari in the late 70s, couldn't remember the operating system, looked it up and it was listed as Default OS, they either didn't call it anything or called it DOS. Learned how to program Basic and assembly, which let him do a lot.
Very shortly after that, jumped into DOS because he got a job for a game company where he was making tools for them on Atari but they were going to make Grand Slam Bridge in DOS so he learned it. They made Grand Slam Bridge, and in making that he learned that with DOS it lets you do anything you want, but you have to do everything, it doesn't really give you much. If you want a pop up window, code everything yourself, want a new font, draw it yourself. It's a thin layer over what the chips on that motherboard provide to you.
In college while working on Grand Slam Bridge, University of Virginia used an OS called PRIMOS. Learned a lot of different flavors of Unix, PRIMOS was kind of a mix of DOS and Unix. University tried to teach a wide variety of languages. Learned C(Sort of already knew that, has a story about something that happened to him in a C class there he will save for another time, Pascal(kind of already learned), Prologue, Snowball, Fortran, etc. Different languages for different purposes, snowball for string, Fortran for Scientific calculations.
Went straight to grad school, they were all Unix. Already a bit used to it thanks to PRIMOS, loved it. Unix provided many useful accessors and functionality, supported Cain's inner hacker, could make his messages to his friends look like he sent him a message From God in Heaven and his friend could reply From Satan in Hell. Had features you don't see in modern OSes. As an example is how they handled files and directories, a directory was just a collection of files, but files weren't just a bunch of labeled data, they pointed to internal nodes and the i-node contained all the information about the file and it's sectors of stored data. That was important because you could have two folders in two different directories that pointed to the same i-node. You could open one, edit it, close it, and go to the other one and it has the edits. Sort of like Windows Shortcut, but not quite the same because both of those files are really the file. Delete one, the other remains. The i-node knows who is pointing to it, and as long as one point it remains it won't be deleted. As an example if you wanted to save a family photo, he can store it in Cain Family, but since it has his siblings, he may want the image also in his photo folders for his siblings. In Windows, he would have to make six copies of the image (1 for Family, 5 for each siblings folder) or use shortcuts, which aren't really the photo. Under this alternate system, it would be five files, same photo. Edit one, they all get edited at once. Try and delete one, you don't lose the ones in the other folder. Got used to this method, got annoyed going back to the traditional system in DOS and Windows.
Going to Interplay meant going back to DOS.
Interplay often bought computers early on to do 3D rendering from Sun , these ran Irix, very strongly flavored version of Unix. When the 3D artists had a problem, Cain would often be brought in to help given he knew Unix.
Cain will not defend Windows, it is sometimes very hard to use and to code for. Two nice things are, like DOS, if you can conceive of what to do, you can probably do it. Also, excellent backwards compatibility, Cain is still playing Windows games from 30 years ago. Can't do that with Apple.
On the subject of Apple, Cain only made one game for it, Fallout, and that was technically just GNW having a MacOS version. In Cain's career, Cain has not seen much indication Apple cares much for games. They occasionally make a half hearted effort like Game Sprockets, but then they disappear. Not even sure they care about gaming on iPhone. Have to do everything Apple's ways, or no way. Fallout had thousands of art files to process, and they were processed separately for DOS/Windows PC. For Windows, they could run a simple batch file that could grab the extensions for the art file and process them through another application. That was way more difficult on Mac, they had to use a third party batch file system and find something that accepted commands. Had to rope the files together in chunks. Mac guy complained they had thousands of art files in one folder, Cain said it was supposed to be like that. Apple people would say if it's something they don't support, you are doing it wrong. Cain's first apple phone couldn't send text messages to multiple people, friend of his who was a big Apple guy said that wasn't a big deal. When Cain tried to figure out how to send a group message to a 12 person dinner group, he suggested sending each message out individually. Never did anything on Apple past Fallout, can't speak much for coding for it.
Tl;DR: Windows won. Cain used it the most, still would probably use it the most, has the most games for it. Runner up would be Unix or some flavor. Gotta go Windows if you want the most options.
(As an aside, will be out for most of the day around the time Cain would post a video tomorrow, so don't expect a summary.)