While I normally record with my home studio which I use Sonar Platinum. But there are times when I will go portable when at band practice. I don't use my iPhone but use a Ipad for this. The app I use is a pro 48 track DAW called Auria. You can find it here.
Auria 48 track DAW
I agree with Mat, it is not the app but the mic and interface to the iPhone. I just recently purchased the IRig Acoustic mic and running it into the Amplitube acoustic app . I then use a app called Audiobus to route the signal between the iRig acoustic app and Auria DAW.

If you are recording Electric guitars I would recommend using a interface to the phone such as what Guitarhacker pointed out or something like this iRig HD . There are lots of mics and interfaces for your iPhone out there.

I think you will be amazed at the quality of the sound you can get out of these devices.

Brian

BIAB 2025 Ultrapack- Studio One Pro 7 Windows 11, Mac Mini M4 with Logic Pro 11, Melodyne Studio