Here's some of the apps that have been really helpful for me this year (I'm on iOS, not sure which are available for Android.)

Happy Scale
https://itunes.apple.com/us/app/happy-scale/id532430574?mt=8

This is a GREAT weight loss tracker which lets you set multiple goals along with milestones that are tracked. The unique thing about it as a weight tracker is that it uses an algorithm to predict how you're tracking with a moving average that smooths out the peaks and valleys. If you are weighing yourself every day, even when trying to control for variables like time, fasted/no fasted, exercise, sleep, etc. there will be variables like water retention or the whoosh effect. This app makes it easier to get an idea of how you're really doing.

Trophy Hunter
https://itunes.apple.com/au/app/trophy-hunter-workout-tracker/id1252224851?mt=8

This is a good app to use for tracking your weight lifting workouts. I've been going through a cycle of incremental progression, and then deloading a bit at higher reps to work on form, and then progressing again. To do this you have to be thorough in tracking where you are at with weight, reps, and sets, and this is a simple, easy to use app for doing just that. There are other apps with more features that I've tried like Strong, but the added complexity made them not as easy to use while at the gym in the middle of sets.

Zero
https://itunes.apple.com/us/app/zero-fasting-tracker/id1168348542?mt=8

This is an intermittent fasting timer app which is great to track your fasting time. I actually don't even use this any more because it's an automatic thing for me now, but it was crucial for the first 2-3 months while I was forming fasting habits. You can set the timer to either forward or reverse to track when you can start eating again. It's also cool that it shows you how you've done for the week so you can track your average.

Rack Math
https://itunes.apple.com/us/app/rackmath-barbell-calculator/id1095054017?mt=8

This one is just a calculator for figuring out which plates to put on the barbell. Seems like it's not a big deal, but when you are increment 5 lbs or so every workout it saves you time from doing quick math in your head to calculate 230lbs or whatever.

Seconds
https://itunes.apple.com/us/app/seconds-pro-interval-timer/id363978811?mt=8

This is an interval timer which lets you customize pretty much everything. I use it for HIIT cardio that I do on a heavy bag. There are other interval timers, but I love the customization options available with Seconds.

Apple Watch
This obviously isn't just an app, but it's been cruicial in my fitness journey. I had a Fitbit before, but IMO just tracking steps isn't a true look at how active I am. The three rings of move (calories burned), exercise (minutes of exercise activity) and stand (hours of the day that you were standing for at least a couple minutes) are a more complete picture of activity. I try to close all three rings literally every day. It's also great for tracking heart rate with averages and cooldown periods for workouts to be sure I'm pushing myself. I also use that sometimes to track my rest periods between sets - instead of resting for a set amount of time I rest until my heart rate gets down to below a certain number. I was a big watch guy in the past with at least 20 different watches that I'd switch out every day, and that held me back from getting Apple Watch for a while. But now I just switch out the straps/bands on it to have some variety.