I have been a user of various personal trackers for years. Went through a few Fitbits, owned a BASIS watch before they were acquired by Intel and promptly scrapped, etc. For the moment I settled with the cheapest Garmin Forerunner, the 235. My wife wears Nokia/Withings (though it's just Withings now) SteelCharge watch and is quite happy with it, and their newer ScanWatch looks like a great product when it comes out in Q2 2020.
My personal requirements are that I don't have to charge the thing daily (thus, no Wear OS watches for me, even though I fancy the Fossil Sport ones going for cheap now), and that it has a user-removable/swappable bracelet, because for whatever reason I seem to wear those out (plus in hot climates you want to be able to maybe swap between steel/leather/silicon sometimes).
Importantly, it's not just what gimmicks a particular device has, but what you can do with the data. Most manufacturers try to lock you into their ecosystem (especially Garmin), and even if not, the amount and character of insights you can get from their platforms/apps is usually limited. Just a bit more than a year ago, I have discovered a small Australian startup called exist.io (disclosure: link is a referral, posting it this way is that if you decide to try it out, you'd get 2 months free trial instead of just one; not that they are expensive at $6 monthly, but still, it's a recurring subscription; not otherwise affiliated, just a happy customer) which added a lot of value to my "quantified self" efforts, and seems to be a kind of small, very transparent and genuine outfit that I like in general, besides the functionality. It allows you to assemble a number of data sources including most of trackers, and give you a very nice set of visualisations to help you with insights. They also try to show you trends and correlations (especially if you use their features to tag/annotate your day regularly, which I find very useful), but not insistently and they don't sell your data to anyone. On topic for this conversation, here's my sleep tracking page there as an example (the two visible correlations are pretty mundane, but there's more of them below which I cut for personal reasons :) and some of them are useful and not always obvious; as usual, if you take time to rate the correlations, they improve faster) - https://prnt.sc/qngozi
In comparison, this a daily sleep details screen from Garmin's own Connect app/website, for a weekday->weekend night - https://prnt.sc/qngwsr
All manufacturers have their own quirks and foci, e.g. Garmin is heavily sports/activity oriented, while Withings focuses on generic health approach.