The list of best altcoins must be started by Ethereum, which is the second most valuable trading currency after Bitcoin. As one of the most popular crypto, you can buy Ethereum in any trading platform.
Besides Ethereum, you can invest in many altcoins. Just learn a bit more about the cryptocurrencies to make your choice. I prefer to deal with NEO, Cardano, and Lisk. Recently, I added Credits, KuCoin, and EIDOO to this list.