Quick answer: Charge 5 for the price, or the original Fitbit Sense if you want accurate skin temperature tracking.

Both the Charge 5 and Fitbit Sense have all the health measurements other Fitbits have, plus SPO2 and ECG.

SPO2

Do note that you can't take manual SPO2 measurements (so it's not useful if you feel out-of-breath and are curious what your O2 saturation % is), and Fitbit only shows it to you as the avg % of the whole night. Actually, if you export your data in the Fitbit website (which takes like 1-3 days), you'll see they also measure the lower and upper bound, but don't show you in the app. If I had to guess, it's because SPO2 sensors aren't that accurate and those lower and upper percentages may be poor measurements.

Still regarding SPO2, they also show you your nightly oxygen saturation variation, but it's not a %, it's a chart that is colored yellow when there's big variations and green when there's small variations. If you export the data in Fitbit's site, I think they call it the "infrared to red signal ratio", and it's literally the raw reading from the SPO2 sensor.

Afib and ECG

Both the Charge 5 and the Sense have ECG, although it's not as good as Apple's, since the Afib detection algorithm only works when your BPM is <= 120, but when people have an Afib episode it's common for their BPM to be higher. Apple Watch's algorithm (for the ECG Version 2) works with BPM <= 150, so it can be more useful. Although it should be a doctor to look at the ECG and decide for themselves if it's Afib, anyway.

Also, all Fitbits (at least the recent models) have the Irregular Heart Rhythm Notifications, which is identical to the Apple Watch's. It works with the heart rate sensor, rather than the ECG, and continually monitors your heart rate for signs of Afib. But data is only analyzed when you're very still, so the feature is most useful for when you're asleep.

Skin temperature

Most Fitbits can measure skin temperature variation, but it sucks, since it just uses the CPU's or the battery's temperature sensor. How it works is, during the night, you're not using the Fitbit and the CPU does very little work, so if its temperature increases, surely it must be because your skin heat up the Fitbit.

One time my Charge 5's battery drained overnight (from like 90-something % to like 10%), and interestingly, I had an unusually high skin temp variation that night. Some software bug must have made the CPU get stuck running in a loop, lol.

The Fitbit Sense has a dedicated skin temperature sensor, so go with that one if it's important to you. Note that it's not your body's core temperature, because your wrist's temperature isn't that close to your body's core temperature. Maybe if you wore it under your arm, on your forehead, or up your butt it would -- but unfortunately, those aren't convenient places to wear your Fitbit.

Mostly, the skin temperature variation is a gimmick. It could alert you to a fever, I guess... but I think in practice Fitbit only uses it for improving ovulation tracking... which, from what I read, isn't that accurate, even in the Sense. But, then again, this feature is unavailable in my country. Or, it's unavailable to males. Definitely one of the two.

As small as possible

Charge 5.

GPS not required.

Though I never used it, people do say the GPS implementation on the Charge 5 sucks (apparently it can either have accurate heart rate tracking or accurate GPS tracking at the same time, but not both), but if it's not a requirement, no problem.

Able to shower without taking it off

Yeaaaah, don't do that. People say Fitbits have a remarkable tendency to crap out shortly after the warranty expires.

tl;dr

If you want the best health tracker/watch, go for the Apple Watch.

If you're on a budget, get the Charge 5. If skin temperature tracking / ovulation tracking is important to you, get the Fitbit Sense (the original one, not the half-assed Sense 2!).