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.