Get out to the bus stop around 6:25 AM, assuming the actual rope drop is at 8.. Buses start around 6:30 or an hour before rope drop whichever comes first. You need to be able to have more than one possible bus to choose from in case the best choice might be too full for you to get on.
I would advise not planning on an Uber or other private car. That could get stuck at the entrance "toll plaza" if you are unlucky to go to Animal Kingdom on a date that no one is manning the plaza that early.
Perhaps a good place to reiterate part of the response to the original post:Will buses run early enough to get us there by 7:45 for a 9am opening?
Buses start around 6:30 or an hour before rope drop whichever comes first.