I think supporting actress will be most competitive.
There are so many
Lead Actor: not a single potential nominee is a lock nor a front runner. We won’t know anything until critics give out their awards and televised award shows give out their nominations.
Sure Sup Actress has multiple potential nominees but if Oprah takes with each televised ceremony as a nominee her category won’t shift much in the lineup of the women right beside her.
As Words+Count said, nobody is a lock for best actor yet and there are more than five performances that have already been seen and acclaimed. My vote goes there.
There are also no locks for Best Supporting Actress (many will claim that Oprah is but she’s not) but far less performances contending in that category have been seen, at least by the masses.