Garmin
Forerunner 255 Series
Long battery; but wrist HR tracking is inconsistent.

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>a lot of smartwatches had to be recharged every few hours, not just every day My Amazfit Bip had about 3 weeks of battery life, Amazfit GTS3 had 2 weeks, and the current Amazfit Bip 5 have 7 to 10 days.
I wouldn't use it for any scientific research. To be clear: I've never used it. I've used the Xiaomi S1 watch (high end), the Amazfit Bip, Band... something, and I have the Amazfit Band 9 Active right now to tide me over because the backlight on my Garmin Vivoactive 3 has died. (And because it cost 40myr delivered ($10usd)) The two bands - Bip have been dubiously accurate. Basically fine, I guess.
I sleep with my trex 3 pro (48mm) which is very bulky. I think that even though it is big, I progressed over the years from the mi Band 3, to amazfit bip, to GTR4 and then to this one. The progression makes it easier
Amazfit bip. Battery lasts 14ish days, does everything an Apple watch does and does the sports tracking. I love mine. I usually use the walking/hiking feature. I hit the start button for the workout and it let's me know when I've hit a mile, how long it takes me to do a mile etc. It can show where I've gone. The amazfit pro speaker is better than the bip when using the phone function. It does not come with cellular. It only has calls coming through with Bluetooth connection. But you can answer calls and talk over the phone using the watch.
I started with a cheap Amazfit Bip from Amazon that at the time was only $30 and did everything I needed (pace, heart rate, GPS, distance, splits, multiple exercise modes etc,). They still have some basic cheap options but also some higher end models as well.
Start with running shorts with tights. This is essential for me. I won’t consider running without them. They reduce chafing around the thighs, making the entire run more comfortable. For the top, keep it simple. Any lightweight, quick-dry T-shirt works well. Don't overthink this part. Next comes the shoes. Start with a basic running shoe until you understand your running mechanics, like heel strike, foot placement, and pronation. There’s no need to invest heavily before you figure out what suits your running style. Finally, get a tracker. If you carry your phone while running, a basic fitness band without GPS is more than enough (Samsung Fit 3, Mi Band, Amazfit Bip). If you don’t carry your phone, then go for a proper running watch with dual-band GPS (Coros, Samsung Watch 7/8, Amazfit Balance or T-Rex series). Hydration becomes important only for long runs (anything over 90 minutes). For shorter runs, it’s usually not necessary.
Garmin
Forerunner 255 Series
Long battery; but wrist HR tracking is inconsistent.

Garmin
Forerunner 955 Series
Best value, clear MIP screen; limited smartwatch features.

Garmin
Fenix 7 Series
Durable, multi-week battery, flashlight; high price, buggy software.
Garmin
Instinct 2 Series
Rugged, solar multi-week battery; bulky, small screen readability issues.

Garmin
Enduro 3
Multi-week solar battery, lightweight; but bulky form factor.

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Oura - Oura Ring Series

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Apple - Ultra Series

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Garmin - Enduro 3

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Garmin - Fenix 7 Series

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Garmin - Forerunner 255 Series

Ranked #1
Oura - Oura Ring Series