IVPN, Mullvad and Protonvpn are the only 3 I would trust they only use dedicated hardware unlike most that use VPSs.
Mullvad is a great service. But their reliance on WireGuard and OpenVPN makes it a less than ideal option on iOS/iPadOS as you have no way of utilising a encryption method that is being hardware accelerated. Performance is not getting affected that much as a result of software acceleration working decently enough, but battery life is a completely different matter. You will see greatly reduced battery life when using WireGuard or OpenVPN on iOS/iPadOS.
Using third-party apps for VPN isn't that great either as it's obscuring battery data and network data usage on iOS/iPadOS as iOS will simply count the app as active 24/7 when it comes to battery and data usage as all network traffic is being routed through this app.
It's much better to utilise services that enables you to use the built-in VPN option within iOS/iPadOS, this won't obscure data so you can still rely on battery usage and network usage data being accurate on a per app basis. A utilising IKEv2 IP-Sec with strong encryption like AES-GCM-256 is just as secure as OpenVPN and WireGuard and do to it featuring hardware acceleration on platforms such as iOS, iPadOS and macOS it will result in better performance and lower impact on battery life.
iVPN is great as they provide you the option of utilising IKEv2 directly on iOS, iPadOS and macOS. You don't have to use their app. Same goes with my preferred service Perfect-Privacy.
Services like NordVPN is more a easy solution for less knowledge users more than anything else. The service doesn't offer much of anything outside using the NordVPN app on any platform.
The reason I prefer Perfect-Privacy is because they offer me to use their service however I see fit. I can use the native option within Windows, the native option within macOS, the native option within iOS and iPadOS. I don't have to rely on their app at all. When I want to just hide my browser I simply create a SSH-tunnel towards Perfect-Privacy and use a proxy to hide my browsing traffic using this encrypted SSH-tunnel. It gives me full control of everything. I can customise about anything for it to fit my needs the best way possible.
Another thing I love about Perfect-Privacy is their support for IPv6. This should be a given i 2021, still it's basically only Mullvad and Perfect-Privacy that are the only one supporting IPv6. You receive both a IPv4 and IPv6 address making you capable of connecting to IPv6 only networks and servers.