*In Shilling section because it’s literally a bunch of links to the Play Store of unendorsed (by site) emulators.
I’ve noticed a lot of people who’ve run into problems due to playing on smartphones and using subpar emulators.
I’ll be enclosing Play Store links and descriptions for various Android GBA emulators I think are neat or at least passable. Only emulators with free versions will be listed.
Nostalgia.GBA
A free and updated fork of GBA.emu, which was a port of a PC emulator. GBA.emu and the rest of Robert Broglia’s emulators used to be the best around. It’s stable, free, has rewind, can be controlled remotely over wifi, and, like John GBA, automatically scans your device for ROMs. There’s a paid version but I don’t remember what it unlocks.
(8.5/10 solid)
My Boy! Lite
A free version of My Boy! GBA. Pretty much the standard for Android due to accessibility, ease of use, and low system requirements. The free version is limited, iirc, to one save slot and to 2x or 4x speed. However, it’s relatively low accuracy and the speed-up mode makes the audio very, um, high frequency. It’s essentially VBA with some jank implementations, which means it also comes with stability and accuracy issues.
(6/10 it’ll do)
*However, I recommend against using this one unless your device is particularly outdated.
Retroarch (mGBA core)
Retroarch’s mGBA core is still the best option for a GBA emulator on higher-end Android devices. Retroarch Plus especially due to supporting 64-bit, which improves performance on, again, higher end devices. Setup is easier than you’d think - just go to the Online Updater → Core Downloader → mGBA. After that, you can go back to the main menu and select Load Content. Your ROMs should go in the downloads folder in the RetroArch folder (check your file manager).
Downloading and installing the apk from their site/github is the best way to make sure your version is up to date, as they don’t push updates to Play Store often.
This is also the only emulator on this list, to my knowledge, to support autopatching! Autopatching automatically applies a ups/ips file to a ROM with the same filename. This is very convenient when you don’t want to accidentally lack a clean ROM or when you want to play Deity Device.
*Please note that the libretro developers have some controversy about them. This is not an endorsement of the developers but rather that there’s no official mGBA port for Android yet.
(9/10 held back by devs not updating Play Store among other things, but otherwise is access to mGBA on your phone)
If you have a 64-bit device, RetroArch Plus supports VBA Next and VBA-M in addition to mGBA, for the occasional spaghetti moment that you need lower accuracy to bypass.
Pizza Boy Basic
A newer option. The UI is very barebones but follows the standards that Qt-based emulators ported to Android have. It apparently generally performs better in terms of accuracy than most other GBA emulators. There are no ads. However, nearly all the features are locked behind a paywall.
(7.5/10, too much paygating and the UI can be difficult to work with)
Don’t see your favorite emulator here? Well, it shouldn’t be your favorite. 
John GBA works fine if you spend the $3 to disable ads, but it’s essentially a cheaper and easier to use My Boy (paid) at that point. All other GBA emulators on the Play Store are pretty… iffy.