Biden. He was elected fair and square (albeit with the help of the Republicans kicking the ball into their own goal).
Note that while Reagan was suffering from dementia, he successfully reinvigorated the economy that was tottering from stagflation after years of “big government” and launched the policies that result in the Cold War coming to a happy conclusion (freeing many countries from socialist tyranny without spilling a drop of blood).
From this we see that senility seems to have little or no impact on the ability of one to be president. The government follows the policies of the leader (which in the current situation, might be less than ideal given that the Democratic platform calls for destroying the private sector, appeasing our enemies and replacing with civil rights with entitlements to politically correct factions).