Bitcoin is not altcoin it is a main crypto coin all the other are altcoins,so I cannot give you answer why it is the best altcoin because it is not a clone coin or altcoin..
Bitcoin is an altcoin because it's an alternative to fiat. Made up categories like "it is a main crypto coin" is more of an opinion not really a reason.
Why bitcoin still the best than altcoin?
Bitcoin has the
first-mover advantage which translated to it's current value. But bitcoin's technological prowess is mediocre at best when compared to other alts in the same category.