There are few cryptocurrencies other than bitcoin that will be Monero, Ethereum, Litecoin and so is preferable for me.
If you mean as important and as popular like Bitcoin then it has to be Ethereum, Ethereum has changed the landscape of Cryptocurrency, it helped launch thousands of new projects because of it's simplicity and many usage, it's the most preferred smart token platform in the industry and they have cemented their success in the industry.