What is the best VPN to use on iMac?

None, Mac OS does not benefit from ANY of the following t types of 3rd party apps.

• Antivirus
• Cleaning
• VPN
• Security
• Maintenance

If you want your Mac to remain stable, fast and above all secure simply use it and leave Mac OS alone. These forums are full of threads from users that installed one or more of the above type of apps and rewarded with exactly what they did not want. A slow, buggy, insecure Mac. Once uninstalled and restored to their original condition the computers started acting how they were intended to. PLEASE do not try to out-thing Apple's security, their software engineers how far more experience than you. If you are a recent convert from MS Windows, please remember that Mac OS is NOT the same as Windows.

I suspect other experienced users will post similar advice if you don't believe me so please wait a few days for their replies.

Part of your question was answered, at > Block my IP address from other to see? - Apple Community

To answer this part of your question..

macOS has built-in security and you do not need third-party Protection software or a VPN.

see > Apple Platform Security - Apple Support

MacOS is the most secure operating system on the market. Privacy is at the very core of all Apple products and services.

I would advise against installing anything that interferes with that.

Don’t be fooled by clever marketing advertisement tactics that rely on fear to sell you a solution to a problem that doesn’t exist.

All the best! 👋🏼😉

Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint.  Read these articles:  Pubic VPN's are anything but private and Security Risks: The Dangers of Using Free VPNs (eccu.edu).    

Additionally a new study ("Apple Offers Apps With Ties to Chinese Military”) is specifically about VPN apps in Apple’s App Store.

You've been forewarned.

None. Using a VPN to access the Internet does nothing to protect your privacy or secure your data. It does exactly the opposite, exposing all of your data traffic to a single entity that can't be trusted.

iCloud private relay is the safest, most effective way to surf the web as it routes all of the traffic from Safari through multiple secure relays. Of course, it can not protect you from yourself if you're prone to fall for phishing attacks and scams. Nothing can do that.