If you are not holding Bitcoin, Ethereum and Tether, I think it is a great mistake, go and check last year run up, Bitcoin and Ethereum out perform the whole Altcoin market, there will be a time that you might also need Tether to buy the dip of Ethereum and BTC, look when Ether was $400 and no one is picking it up at that price, funny isn't it
Personally, I use altcoins to accumulate more BTC. Because of this. BTC is king in this market and will continue to be for years to come. It will rise to triple digits imo.
That being said, there are a few projects that stand to be worthy of long term holds. VeChain is my biggest long-term hold. Been holding since 30c and will continue to hold. Those masternodes will pay my through retirement. Honestly the best project in terms of fundamentals. Jim Breyer as an investor? PwC incubator? Parternships out the wazoo and Sunny Lu handles everything with grace and elegance. They got style man, and I guarantee they will be Top 5 over the next year.
Besides VEN, I'm big on XLM (and maybe Ripple by association) for similar reasons. The market share to be had is large, and their target audience: the 2 billion unbanked.
A long long long term hold you should consider is FIC Network. They will be facilitating mass adoption through fixed income markets. Great for institutional money, which is ultimately needed to drive the next major bull cycle.