Of course Bitcoin is number one because it is the world's first ever digital currency. For now, it is the only gateway towards converting to fiat currency. It is why lots of altcoins cannot topple Bitcoin for a very, very, very long time. Unless Ethereum and the rest are going to step up as alternative gateways for fiat currency conversion. Just my two satoshis.
Exactly, Bitcoin is number 1 because of its convenience towards converting digital money to fiat , It is also accessible for every users who are using bitcoin as their payment structure, Very volatile price that the bitcoin has but it is normal now a days, It is also the most used and first digital currency worldwide, It will soon become to be considered as one of the permanent currency in the world.