I know it's a tricky question because some person don't have any problem, or other have some different from others, or some of us don't even use so much the Inspire so to notice any bug...
I personally (luckily) never had any problem (yet) with my Phantom 4 Pro and Inspire 2, but I was interest in know with some answers, which fw could potentially be the most reliable..and how many fw we can skip to eventually downgrade.

Most reliable fw for Inspire 2?  IMHO - the .300 will qualify as "reliable".  It's not perfect, but of all the fw available, this is the best option.

The .300/.400 firmware as well as the .500 firmware are both dangerous. According to DJI a new firmware release is in the works. See the .500 firmware release thread for more information. In short, .500 firmware fixes a dangerous batter level reading bug. But the .500 has a new "feature" - power throttling, that can happen when you least expect it and turn the I2 into a sluggish drone that in slight winds can be more or less uncontrollable hence making it an hazard.