The Jan 2021 Opensignal 5G user Experience report is out
If you’re looking for video experience on 5G, then Verizon is evidently your carrier. For literally anything else it’s T-Mobile. AT&T does not appear to have won anything (looks like they’re #2 in a couple of categories).

There’s an explanation of 5G video experience and 5G user video experience, but this seems to mostly be with packet prioritization meaning if T-Mobile wanted to change some routing tables / QoS they’d win across the board. My guess is if you’re a live streamer who needs perfect second by second video when you’re not at your computer this might affect your choices.
You can read the full report here. As with anything take with a grain of salt. They do have a pretty good interactive thing down at the bottom which shows experience by state, so before you go calling BS check your state for being abnormal – Tennessee for example AT&T wins download, 5G Video Experience but not 5G User’s Video experience.
So yeah, overall T-Mobile looks to be ever so slightly better on the 5G.
[OpenSignal]