I try to use the default Apple apps as much as possible, and have very few third-party application on my iOS devices.
Here is a list of the non-Apple apps I use:

Overcast: The Apple Podcast app is objectively terrible. Weirdly designed, (with layout and navigation oddities), the intended user seemingly collects podcasts like Pokémon, but never listens to them. Overcast works perfectly, and the Smart Speed feature is impossible to give up.

Wipr: I’ve tried pretty much all of the Content Blockers on macOS and iOS and Wipr is hands down the best. Small, fast, does the job.

NetNewsWire: The RSS reader Apple would make, if Apple cared about RSS anymore. Free on both macOS and iOS. Syncs via a bunch of services, including iCloud.

WhatsApp: Used under protest. Held out for years, but no-one uses Signal, and what’s the use of an empty contact list? Maybe Apple supporting RCS in a future release will negate the need for WhatsApp, but Meta's messaging platform is pretty much the standard in the UK and across Europe.

QuickScan: Life is digital first for nearly everything, and only the occasional- usually government document- is still paper based. For those times, QuickScan is my go-to despite document scanning being a built in iOS feature. QuickScan has on device OCR and is fully private.

iComics: I read a lot of comics, (mostly crime & sci-fi works from Image Comics; not into the superhero stuff from DC and Marvel). iComics supports file formats like CBZ and CBR, saves reading progress and correctly organises series. This app is simple and does what I want it to do. Supposedly a Version 2 is being worked on, but given it was announced in 2020 and no sign as yet; it’s unlikely to happen soon.

Also; my banking application, Waitrose, and a couple of games.