Blue
Ravenous Bugblatter Beast of Traal
The -5/+10 mechanic is broken for a number of reasons, frankly dont know how it got through playtesting. Best solution is to make GWM a "half feat", delete the -5/+10 part and replace with +1 Str.
For a meaning of "best" that means "very unlike the original feat so it doesn't fill the original design space and we need to come up with another feat to do so".
Actually, add in "and is underpowered compared to other feats".
On reconsideration, I thing the word you are looking for is "sub-par". Spending a feat to get half the ASI (and it's still STR focused so it doesn't even open it up to finesse or others) to get "on a crit or kill get a bonus action attack" is pretty weak. It's like "best", but actually reflective of the suggested change to GWM.