What are the best game design software?

Are you more interested in the visual aspect of game design, or the engineering side? Either way, I'd recommend Unity. It's a free game engine, and there's a lot of documentation. It uses either Javascript of C# as coding languages, and I'd recommend you learn some C# if you're interested in game design from the engineering perspective. If you're interested in the artistic aspect, Photoshop is a tried-and-true tool for concept art, and Maya is excellent for 3D rendering (Blender being a solid free alternative).

Best game design software for game development and designing would be either Unreal or Unity 3D.

I have used and created a game on Unity 3D and I did not had prior knowledge of coding or how to use the Unity software. So if I had to say, Unity is perfect if you want to try a game creation software. The website has a bunch of tutorials and how to write in C#.

If you're trying to more a game character designer or 2D/3D artist, there are open 3D creation software. The program I fiddled around with is Blender, the reason why is, it is free and accessible. Maya is the highest tier of 3D creation software but expensive.