I would recommend NEM and Ethereum.
Both NEM and Ethereum are good altcoins to buy right now. This applies specifically to NEM, as it's dirt cheap right now considering all the advanced features it has within a single platform. For the knowledge of many, NEM can also execute smart contracts like Ethereum with its API. The only difference lies in the way such smart contracts are executed, as they're not fully executed within the NEM blockchain (having little to no effect on the network's performance), while in contrast, Ethereum executes everything within its blockchain (causing high load and affecting the network's performance). With NEM, if a smart contract is broken or full of bugs, you can easily repair or upgrade it without affecting the whole NEM blockchain.
Considering all this, I think that NEM (XEM) is quite undervalued right now. The upcoming Catapult scalability upgrade (similar to Lightning Network and Raiden Network), will further boost NEM's ability to process a large amount of transactions per second, without causing delays or disruptions. In its current state, NEM can handle a lot of transactions per second, and it's blazing fast in confirmation times. It's unique PoI (Proof of Importance) consensus algorithm is far more decentralized than most popular alternatives such as PoW or PoS. As such, this is a coin you wouldn't want to miss investing in it for the long run.
Ethereum by the way, is also a good investment these days, due to the recent decline in price. There's a lot coming to Ethereum in the future, which will make it much more scalable and energy efficient than what it is right now. With a change in consensus from PoW to PoS, you can make a lot of money in the future if you buy and hold Ethereum in your wallet today. Other altcoins which are great buys right now are Constellation (uses DAG consensus algorithm), IOTA (DAG cryptocurrency with focus on IoT sector), and MonetaryUnit (has Masternodes, Instant transactions, and will switch from PoW to PoS soon). All in all, it's best to do your own research across cryptocurrency markets to decide on which coin(s) you're going to invest into for a long time.