Activity tracker….

Overgeneralizations:

If your real-life friends have a particular brand, and you like doing challenges with your friends, get the same brand.

People who are very Apple-centric in their other electronic devices tend to like Apple Watch for its smart watch functions and integration with the environment.

Multisport/multi-activity exercise-centric people often go for Garmin. They have some good built-in features for sports, and also quite a list of integrations with other sports-orientate apps.

Fitbits have a little worse reputation for dying young compared to some of the other major brands.

Major issue: What features do you need? Is your main goal MFP integration for calorie estimation, or something(s) else?

I use a Garmin, recently moved from Vivoactive 3 to Vivoactive 4. I'm an on-water rower, and it has some good performance tracking features for that. For rowing specifically, I need it to integrate with a chest belt for heart rate, because there's too much arm flexion for a wrist-based heart rate monitor to handle. I wanted a device that didn't require me to carry my phone when working out. (I don't like taking a phone out on the water, or having it in Bluetooth range every minute for some other activities.)

I'm old, used to wearing a watch, still prefer one for time (vs. checking my phone for time), and I need a big time display because my eyes aren't that great. I'm kind of oblivious, so I need something tough/durable, but as a li'l ol' lady I'm OK with a sports-watch look but don't necessarily want something that makes me look like a wanna-be Navy Seal/Commando. Long-ish battery life is a plus. I didn't think I needed smart watch features, but I like that'll notify (wrist vibrate) if I get a phone notification when in bluetooth range: It means if I'm expecting an important call/text, I can put my phone in my purse/pocket in social situations but not miss the important message.

What for specifically is important to you?

I have a rather expensive garmin watch. Don’t care about smart watch functions, want long battery life and their numbers work for me mostly. Note, this IS individual and might not for you. Also, like other devices their walking calories are shite

I like my Apple Watch for closing rings, but I don’t trust the calorie estimates. I don’t exercise really but I do walk and bike ride. I don’t increase my calories for a walk. I will increase my calories for a bike ride, but not all the calories reported by my watch. I think they’re over-estimates when compared to other things, like the computer on my bike that has a power meter.

I like the Garmin platform for running, so that’s really all I’m familiar with. It does have some capability for recording many other types of movement. Strength, biking, gym cardio equipment, and others. Does everything I’ve ever asked it to but I’m pretty basic. I have the Garmin Instinct Solar watch. Good luck!

I have an Amazfit Pro, it wasn't a brand I'd ever heard of until I randomly came across it in a magazine. I'd been thinking of getting a Samsung one as I have a Samsung phone, but was worried I'd scratch it and the battery life was a concern to me. This one cost me £45, battery lasts a couple of weeks, and itdoes everything I need it to (and yes I have scratched it lol). It's not as all singing and dancing as some and I don't think it's waterproof but it suits me and I know it won't cost the earth to replace if I break it.

I'm very happy with my fitbit Charge 4.
It has a long battery life, and I find it very accurate with steps, cals and HR. It is also small enough and it has added features that I didn't find important until I started to use them: alarm clock, timer, notifications and most importantly a hourly reminder to move.
When it dies, I'll by another fitbit no doubt.

Another Garmin user here. As @Jthanmyfitnesspal said above, the Garmin Fenix series devices do everything most active people want, particularly if you're a serious endurance type. (Lap swim or open water, bike indoor or out, run indoor or out, hike, etc.) Connects to power meter for cycling or running, syncs with Training Peaks, MyfitnessPal, etc. Garmin's other multisport offerings offer many of the same features. (945, 955, etc.)

If you're not looking for all the heavy duty training features, there are other options out there that might make more sense.