The software I mostly use at work and when I freelance is Zbrush. It’s a really powerful software that allows you to work like with clay in traditional sculpting basically, but much faster like everyone says. The possibility to handle millions of polygons and the fantastic and unique features of this program are the perfect combination for people like me who love creating characters but it is also used a lot by environment artists and concept artists. You can do sketches, illustrations, game characters, 3D models ready for 3D printing and much more with it.
The other tools that I use are Maya, 3D Coat, Substance Painter, Keyshot, Photoshop and Marmoset Toolbag. Each of these softwares is great for particularly reasons and purposes but basically are the best or the most used in game development and concepting.