Bitcoin was first and with that brings many things like a development community, integration, awareness among the general public...etc. Now, BTC is a terrible coin in my opinion, it is slow, expensive, tx fees can be outrageous, no privacy controls are in place. BTC is number one right now, even if it is not the best and it will probably stay in this position for some time. I do think that eventually another coin or token will take the top position.