I would probably pick ethereum, monero and litecoin as the top 3 altcoins simply because of the fact that the three of them are very popular and are based on unique ideas.
Ethereum is currently the most used platform for hosting ICOs and can show really good amount of growth in future. It also has a strong development team.
Monero is a coin based upon the idea of privacy. the transactions are secure and untraceable. The Identity of sender and receiver is kept private along with the amount sent during the transaction. You can expected price to be above $300 within 3 years.
Litecoin is a coin known for its fast transaction speed. it is almost 4 times faster than Bitcoin as it has a block time of 2.5 minute.  you can expect its price to be above $200 within 2 years.