I have anxiety and issues with my hands wandering if they're not occupied (resort to nail-biting, finger taping etc), and I've only ever had one stress ball in my life that I've been using for about 3-4 years now but it was too firm for my liking. So I'm not an expert in judging these kinds of things, but this is just my experience;
This set from PlayTherapy is EXACTLY what I was after when I went looking for something to replace my old one. They fit in the palm of my hand, are soft/easy to squeeze but have what is, for me, the perfect level of resistance (an excellent middle ground from my old one), great shape retention so far, feel durable, and love the colours and find the texture of the ball very pleasant. 10/10 would recommend!