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Series 4. Finally makes an Apple Watch a decent rudimentary health monitoring device.
 
The title is a bit limiting in saying “was” …
Most groundbreaking was the S4 but my favorite is my current one, the Ultra 2
 
My favorite was probably series 4. Smaller bezels, speed was good. Felt like a big, modern leap over the others.
 
Nothing is as groundbreaking as the original one.

But if you are asking which was the biggest improvement over the previous model, I'd say around S4-S6, the new releases started feeling more incremental than big improvements. The biggest leap forward was from the original to the S2.

After that, the Ultra was the most dynamic change, but it's also kind of a separate fork. It's too big for slightly-built people like me. If they ever released a smaller size Ultra, I'd jump on that, and call that the best improvement ever since the original watch.
 
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Even though my first Apple Watch was a series 5, I'd have to agree with the OP that the series 3 was most groundbreaking. I remember at the time being shocked that the tiny battery in an Apple watch had enough juice to power an LTE cellular radio.
 
I switched from Series 2 to Series 4 and that was the biggest transition in terms of everything. That was brilliant. Later I moved from 4 to 7 to 10, but didn't have that feeling anymore.
 
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I still think the Series 3 looks a little smarter with the sharper corners but the Series 4 beats it in utility. Time, Payments, Steps and Siri is all I need.

I'm not sure anyone can say anything after S4 given how little the device has changed over the years!
 
For me Ultra was a game changer. It enabled 24 hour + operation on a single charge (even for a full day of activity) and made Apple Watch battery life a non-issue for me.
 
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I liked the earlier AWs but I love the AW Ultra. Mine is AWU v1. [For me] the AW Ultra is 3x as good as the regular AWs.
 
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My first Apple watch was the Series 5. The reason I decided to get it was the Always on Display. Cannot imagine buying an Apple watch without Always On. Love my Series 10!
 
Easily the Ultra (I've had Series 2, Series 5, and keep an SE2 as a backup watch.) For two reasons: first, the battery is probably the minimum size battery Apple Watch should have had from the beginning. Second, the Action button makes things so much easier for recording workouts - specifically, there is no more 3 second countdown (yes, I know, you can tap the screen to stop it - doesn't always work if you are wearing gloves), so the action button allows precision start, plus it's much easier to pause a workout pressing the action and side button at the same time rather than the side button and crown at the same time.
 
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