What is the best VPN app (free or otherwise) for iOS if I am looking to make my phone more secure?

Free is obviously preferred but from what little I’ve read it seems like the free services that show ads are selling your browsing history to generate revenue, which seems to defeat the purpose (unless you’re just using a VPN to access blocked sites).

I am currently testing VPN Master and Cloak. Any thoughts specifically on these? The free version of VPN Master shows ads. I am however open to all suggestions.

I’ve used VPNs previously for school and work and understand what they are at a high level but am unsure how to judge how secure an iOS VPN app is.

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