Biden is having typical elderly cognitive slow down, but I see no signs of Dementia. Trump actually was given a dementia test, so we can assume that doctors thought he might be showing signs. I think his mental health problems and general unfitness are something other than dementia though. I do not know anyone in office who specifically has dementia, but there are several who lack either the intelligence or sanity or both to be fit for office.
Since the only other defeated president still alive is Jimmy Carter, I will assume that he is your favorite defeated president.