There are several 'bests'. Pick one you can afford, ideally matched in OS to your smart phone, and you're on your way.
The one's consistently highest rated are the Apple and the Galaxy. I own a Galaxy that measures all the usual stuff, and I read that it will report blood glucose in the next issue, coming up in May-ish. Note that not all jurisdictions will approve that app until it has been validated. Here in Canada, I had a Galaxy 4 that needed about eight months' wait before our government gave it a thumb up. Then I got a notification, an update to the app, and the rest is history.
My Galaxy records sleep stage and duration, SO2 sat while sleeping, does an ECG every ten minutes, records lowest HR and highest, and I can take my BP when I awaken to see how rested I am. I have a slight spare tire around my waist, so I will be grateful to check blood glucose when the time comes.
One thing you should keep in mind, though, is that 'constant' monitoring, where you desire an alert, will drain your watch's battery at about double the rate. I set mine, now that I am AF-free, to only measure my HR, and that is whenever I move from seated....or it takes one every ten minutes when seated/resting.
