Which Garmin watch (es) have the most accurate heart rate monitors, or are they all the same?

If you rule out old models, then all use the same heart rate sensor. However, due to physical properties of some watches - like for example the F6 being large and heavy - the hr sensor might have a harder time to get accurate reading due to the watch bouncing around on your wrist due to its weight. So in theory, a tightly worn light watch can give you better results than a heavy watch, even if both use the same sensor.

As a basic principle I'd agree with that. Small watch better than large, light watch better than heavy.

Wearing the watch in the right place on the wrist and with the right strap tightness is also important.

I do my best on the second part, but I find my 6X Pro Solar OHR to be less than perfect even for casual walking and I now use a chest strap for walking the dog, hands free with the dog off lead. Otherwise I can get too many shocks when I glance at my heart rate and think my fitness is in the toilet. Of course I use a strap for cycling and running.

It is probably good enough for sitting still.

My Fenix 6x pro was pretty good, worn as tight and as far away from the wrist bone as possible, both for running and for walking and every day wearing. After firmware 19.20 the OHR went down the toilet. It is useless in any scenario. I am wearing an apple watch on my other wrist so I can compare the measurements and I have been wearing the apple before 19:20 too.

For me the WHR has been useless for the last few months. Had the F6 since it came out. First 12 months or so was excellent. Now I might as well turn off the Hr sensor when running or cycling. Totally useless. Bought a Polar HR strap now suffer constant ANT drop outs. Loosing faith in Garmin after using them for years. 

Venu 2 and Forerunner 945 LTE have the newer elevate 4.0 heartrate sensor, which provide better accuracy overall. But in the end, if you really want accurate data, always go für a cheststrap.

The most accurate heart rate monitor is an external HR chest strap.

Every time I tried using built-in optical HR monitor when running, either on Fenix 6X or Suunto 9, they failed sooner or later, typically by showing me unreasonably high HR for the level of effort. For example, I may be running with an effort that I know should be around 135-140 bpm, and the watch is showing me 155-175. That is a clear indication that it has latched to cadence, and that happens on pretty every single run that I try to use the built-in optical HR sensor. I believe that is caused by bouncing because I am much more likely to see that on a downhill. That is also much more likely to happen on a cold weather run when blood flow to arms gets reduced. 

Get an Apple Watch if you want accurate HR. I just replaced my AW with a Fenix7X Saphire Solar at well over a thousand dollars and the HR monitoring SUCKS. All the Garmin hype is BS, I’m super disappointed. All the workarounds and excuses are absurd, the HR drops for mins at a time, this isn’t a fit issue. It’s a bug and needs to be fixed.