Over my past three years of experience with design so far, the tool I've used and relied on the most would definitely be the Adobe Photoshop program.
Photoshop is very easy to use because of the ease of access that it has for new users due to it's credibly intuitive design and layout as well as the in-depth tutorials and tool-tips that Photoshop provides. As well, the program can be used for a number of different design practises.
Not only is Photoshop incredibly easy to use, it also allows for a mass of different uses for all levels experience. After using the program myself for three years there are still plenty of features that I am still learning and am constantly learning new ways to use the techniques I already know in incredibly new and innovative ways, as Smith (2019) described, "Changing backgrounds, simulating a real-life painting, or creating an alternative view of the universe are all possible with Adobe Photoshop".
These features are also being continuously updated by Adobe with new versions of the program consistently coming out and introducing even more useful features to improve the program. This can be troublesome however as with every update the program gets increasingly larger as they continue to add more to it.
The program is unfortunately very hardware intensive and uses quite a bit of RAM and processing power to run, so a fairly powerful computer is recommended to use the program efficiently. The only other reason not to get the program as a designer would be the large price tag associated with it as well as all Adobe products. The program is capable of a lot of similar uses as other Adobe programs and can be used as a kind of general program for digital creation instead of having to pay for all of the Adobe programs. If you're a student you can potentially look at discounted prices for the Adobe Suite offered by some schools.
References
Smith, J. (2019, Decemeber). What Is Photoshop. American Graphics Institute. https://www.agitraining.com/adobe/photoshop/classes/what-is-photoshop