All depends on your skill level really. From my personal experience I think Photoshop is by far the best but I also know that many other people find software like GIMP to be great as well If youre struggling to get a copy of Photoshop id try that
There are tons of free apps and site you can use. I have never used Photoshop and i am a Photoshop user with certain things. Since i started my channel i have used several different free sites. the one i am using currently is https://pixlr.com/express/
There is also a free app for iOS and android as well as a free desktop download for windows and MAC.