Running

Here is a somewhat recent article from a guy who reviews these things incredibly in depth. He tests 6 Garmins and a Coros here simultaneously and compares their accuracy. DC rainmaker gps comparison test

It seems the cheaper Garmins aren't quite as good in the more difficult city area but the stuff out in the open stuff looks pretty comparable to the more expensive ones. Much of it seems to be multiband vs. not.

If you're into a specific watch he probably has a review on it. Very in depth and I believe gps accuracy is something usually covered.

Last year I upgraded from a pretty old Vivioactive 3 to a 955 with a multiband GPS. The difference in GPS accuracy was definately noticable but didn't really end up making a huge difference overall. The main practical difference is that if I am running in the open and at a constant pace for at least the last 15 seconds, I can see my current pace good to within 5 seconds per km (most of the time). With my old watch the current pace reading bounced around too much to really be usable, but the average pace over the last km or half mile was usually fine. My old watch would occasionally show a track way off when I was in trees or tall buildings and the new one doesn't seem to do that, but that never happened enough with my old watch to bother me. Other than this it is really just the difference between seeing on the map afterwards which street I was on versus which side of that street I was on; the 955 is a bit better but the old watch was just fine for most uses.

If you just want to see how far you went and your average pace, I think any of the newish Garmin or equivalent watches are just fine. If you really decide want the best GPS look at the ones with multiband GPS receivers (that is ones that pick up several different GPS frequecies as opposed to one GPS frequency plus Gallielo and/or GLONASS).

My wife bought me the Forerunner 965 for Father's Day and I am really happy with it. Right now, I think the Forerunner 265 is the best bang for the buck. I wanted the larger screen and onscreen maps the 965 offered. Both offer multiband which offers the best accuracy and auto select which will decide the best gps mode for accuracy and battery life. As others have said, DC Rainmaker does really thorough reviews that can help you decide.

I'm not a runner, but I've seen good advice on Smartwatches from this YouTube channel: https://youtube.com/@TheQuantifiedScientist. He goes extremely in depth. Not sure if he covers GPS specifically though!

Currently using a coros apex, and very happy

I just got a fenix 6 pro sapphire used on ebay - honestly I'm in love with it. I def recommend it! For youtubers - try DCRainmaker

I think forerunner 265 and 965 have L1 L2 GPS, plus SatIQ Technology