You can invest only in bitcoin if you don't want to invest in other cryptocurrencies. If given the choice, I would just invest in bitcoin as I am almost sure that it will exist for a long time compared to other cryptocurrencies that are only use for speculation purposes. There are other coins that has a real purpose of solving the current problems with regards to transactions to make it more efficient and effective but bitcoin has been the very first cryptocurrency and it retain its position because many still believe in it