There are 3 main apps I currently use to manage my health & fitness. One for nutrition - MyFitnessPal, one for gym activity - Gym Genius, and one for overall health and monitoring - Lark. Each one of these has a very specific job, which is why they all fit so well into the way I want to manage my health & fitness.
Let's start with MyFitnessPal. I use it strictly to count calories. With apps for iOS (iPhone & iPad) as well as Android, you can get access to your information just about anyway you need it. The true beauty though is the built in barcode scanner which allows a user to scan the barcode on the back of any food product and instantly add it to their daily food diary. This has been the sole reason I can manange to keep up with this for the last 60 days and counting. It also transfers it's data to Apple's Health Kit app so you have a copy in case you ever want to get rid of the app or try something new. I've tried a few others, LifeSum for one, but they're either too difficult to use or require a subscription for anything other than a few obvious selections. The fact that Under Armour puchased myfitnesspal does have me a little concerned about the app's long term viability, but at least it wasn't Google who purchased them.
Next up is Gym Genius for tracking my workouts. I've been in and out of the gym for over 20 years. I know what I'm doing and I rarely do any cardio. The main function I'm looking for is an intuitive and flexible way to record my exercises, sets, and weight and Gym Genius is proabably the one app in the app store that does this very well. A rest timer is a huge bonus and so are some finer details like personal records. But what I absolutely don't want is an app that tries to sell me a routine or try to provide me with exercise examples in the app. If you don't know what a military press is, google it.
Lark is the last in the trio and the app I'm least familiar with because I've only been using it for about a week. But in that week it's made quite an impression on me. The app itself is setup as if you're messaging back and forth with the computer inside the app. Each time you open it you begin a new conversation with the A.I. This is a great way of interacting with a user, but what's making me stick around is the Activity section with it's List & Chart views. Being able to clearly see all the activity Lark is recording and even more important, able to change that data easily into exactly what it actually should be is very handy.
For instance, because there really isn't a smart app or mainstream hardware for gym rats like me yet (hence all the reason I'm still using a manual app like Gym Genius) Lark tends to record my activity in the gym as a "Walk" instead of a "Workout". That's not too big of a deal though thanks to how flexible and editable Lark's data is to the user. And all that data is also transfered over to Apple's Health as well.
In the End…
Obviously my app trio isn't for most people. I have a very specific goal in mind and I'm working to achieve those goals with specific apps. Another resource is MacStories' touching, albeit looooooong, review of how he's using fitness apps to overcome his treatment from Cancer. In the end it's really about finding apps that work the way you want them to work so you can maintain your routine for longer than it took to read this article.