What is the best VPN for iOS devices?

In most cases, None. There are only a couple of uses cases for a VPN:

• To manage private work networks
• Access internet when in a restrictive government
• Possibly using Public Wifi/Hotspot

If your concern is Security, then a VPN will not provide that for you. All of your web data and traffic is transmitted to the VPN company unprotected. Many, especially the free ones, will sell your data for another income stream. You would be better off using the built in security of the device. For web browsing, your data is currently encrypted as long as the site is using https and you will get a warning if you try to visit a site that does not follow that security protocol.

If you still think you need one, I would research it thoroughly including using the Search option on the top of this page and enter the name of the service you are looking at. This will detail the issues that have been found with using that service. Most likely you will find many issues in these forums.

The best VPN is NONE. VPN is not only unnecessary on iPhone, but it can cause issues effecting the normal operation of iPhone. iPhone is end-to-end encrypted, making a VPN totally worthless. Everyday, someone finds their way to this forum, claiming things aren't working as expected, such as location services, or they can't update their iPhone and when we ask if they have a VPN and they say they do, we tell them to remove it and they learn things works as expected again.

I am looking for suggestions for an effective (best) VPN solution for my iOS devices.

None, unless your workplace or school requires this to connect to their network servers. If that might be the case, the IT department will provide the VPN software to you.