I guess I'm bumping this with my first ever post in this forum...
I use an Android phone so this is not neccessarily for iPhone users.
For weather I suggest Windy. It's by far the most elaborate and effective weather app out there. Windy.cz is a Czech company that now has fused with MeteoBlue, a Swiss weather service that is arguably the best on the planet. I am a hot-air balloon pilot and use these two (Windy on the phone and MeteoBlue on the computer) to prepare my flights. Windy in its basic form is free, with no other ads but one popup if you start it to entice you to get their Premium version. The basic is fully functional but the Premium adds some things such as more models and and hourly predictions instead of per three hours. Highly recommended.
In addition to that, for free mapping, there is Mapy (with one p), also Czech and from the same people. Again, free, using Open Street maps. The two can combine to provide you with accurate weather en route info when you use Mapy for car navigation. Handy. Maps can be downloaded for offline use.
Having a true OS map app is always useful, even though most of these come with an annual subscription. I use the French one as I live in the country of cheese and wine, it costs me 20 euros for a year, ergo about the price of one paper map sheet, to have digital maps of the entire country in all the scales and versions they have available, including historical maps, aeronautical maps and so on. You can download areas you want to hike in for offgrid use, too.
Some app to help you recognize plants and maybe animals (traces) have been suggested already. And depending on the phone, a flashlight app if it's not already baked in, maybe a better camera app to replace the baked in one. I think Google/Android has a Safety/rescue app in its most recent OS versions, otherwise one of those might possibly be found in an app store. Having good emergency info on your phone that rescuers can access even when the screen is locked is a good idea - it will allow them to call a pre-programmed person of confidence, find medical data such as blood type, allergies, medications, implants and such to avoid mishaps in an ER, and of course know who you are.
I also have a red cross app that gives first aid instructions (I have a first aid certificate as well, the app is basically the curriculum for that).
Commercial or free tourist-office apps with marked hiking trails may be handy.
Podometer; heart rate monitor...
On the phone itself - I use a Crosscall X4 rugged outdoor phone, designed in France. I think they are also on sale in the UK. These are intended as 'lone worker' phones on construction sites, or doing maintenance on infrastructure out in the sticks. Shockproof to mil standards, waterproof to 2 meters in sweet or salt water for 30 minutes, reinforced frames, antishock layer behind the screen and so on. They have a programmable emergency button with a baked-in Emergency rescue app behind it, that will automagically call rescue services or any other preprogrammed number, send also an SMS message there with your GPS coordinates, switch on the handsfree speaker/mike so you can talk without needing your hands, strobe the flash to make you easier to find n the field and so on. Basically, the phone is designed to survive a fall or other violence that will hurt but not kill the user, so one can call for help. They come with a clean basic Android interface and a few small apps to test their sensors and such (meaning there is a compass built in). No bloatware baked in. I like them. Half the price of an iPhone or high-end Samsung, roughly. And if mine gets really dirty I can just wash it under the tap. Seems useful for someone out in the woods. One does not need a fancy smartphone with a full-glass body.
No link, easy to find by Googling them, I have no commercial interest in them.
