As others have said, make sure to choose a VPN that does not log your activity. One of the points of having a VPN is that you want your connection history to be private.
I have an account with Strong VPN which works well for me. One of the driving factors in my choice is that the same account can be used on iOS or macOS, and you don't need to install any software on your device. You can simply define a VPN iKEv2 configuration in network settings and set up the connection credentials. Once that's done, connecting is as simple as clicking the VPN switch in iOS Settings, or selecting the VPN status in the macOS menu bar. The last thing I want to worry about is the security of "VPN apps". Better that the operating system can take care of that itself.