I use the Flydigi Apex controller and love it. I’m a mobile gamer, content creator and streamer and was looking into mobile game pads for a while. I did a ton of research before settling on the Apex. One key thing to be aware of when looking at mobile controllers is they are not all the same. Some require 3rd party add on apps that will get you banned in some games. Some require you to play different versions of the game by launching it from their own (not always legit) game launcher.
a big thing to consider when looking at mobile gamepad controllers is how you will be using it and with what games. Now that iOS supports controllers natively now, there are a limited number of games that also support them. However, if you want a lot of control and to use a controller with any game, you will want one that has full button mapping. This is where most if the controllers will vary. The one I use, has an app that’s available on the App Store that lets you map any button, and it can get very in depth with custom macros and such.
Anyhow, good luck in your search and hopefully you find something that works well for you.