Shopping for toothpaste can be overwhelming. There's a lot of brands on store shelves and many claim to do the same things, like prevent tooth decay, freshen breath, tooth whitening, and reduce gum bleeding.
I believe that you should get a toothpaste that helps you with your specific dental problems. If your check-up visits with the dentist are uneventful and you don't really have any major problems, then you can probably use any toothpaste you like. But if you have specific dental problems, then which toothpaste you use can make a big difference in your oral health.
So if you you keep getting cavities, first you should read this article on why you get cavities and what you can do about it. There's more to tooth decay than just what toothpaste you use, so please make sure you investigate the other causes, like diet and flossing.
Okay, so if all you really need is to fight tooth decay, I recommend Crest Cavity Protection, which has 0.243% sodium fluoride. Please be advised that this toothpaste contains sodium lauryl sulfate (SLS), which in rare cases can cause canker sores.
If I have a patient with major tooth decay issues, I will prescribe Colgate Prevident 5000, which has 1.1% sodium fluoride. This is only available by prescription, so you'll need to discuss this with your dentist. This also contains SLS, so you're dentist will recommend discontinuing use if you're one of the rare folks who get canker sores from that.