It really does depend what you want.
I have a flex2 - it's slim, not very noticeable and waterproof to swim in (that was the main selling point for me since I often forget to take watches, etc off when I shower or go swimming). However it doesn't have a display screen just 5 little light up dots (the more dots light up the closer you are to your goal), so you have to log into app to see stats. It also doesn't measure floors or heart rate.
The 'one' is good if you want a clip on rather than wearable tracker. It's around the same price as flex2 and measures floors climbed unlike flex2 but is not waterproof. It has a screen giving basic stats. It doesn't measure heart rate.
Alta basically has same functions as flex2 but not waterproof and has a screen for stats and watch. Does not measure floors climbed or heart rate.
Alta HR same as Alta plus heart rate.
Charge 2 is slightly bigger than alta (but still pretty slim). It does pretty much everything the average person could want (but not waterproof) - floors climbed, heart rate, screen for stats / watch, etc.
All the trackers track active mins (minutes doing moderate or high intensity activity), active hours (hours where you have done more than 250 steps), steps, distance, with phone on you can map walks /running routes you do and sleep (I think the heart rate ones can track your actual rem cycles now but the non heart rate ones can still track when you are restless and when you sleep deeply).