Well, if I may I would like to join the discussion. All 8 listed above are good privacy coins, but do not forget about Cloak coin as well, old good established project, just small comunity and dev it seems its one man show.
But in general if we speak about what coins to buy and hold, I would look at more early entering for better ROI. To buy dash today is 1 thing and to buy 1 year ago is another thing. I entered Dash at 40$ very nice profits. I am holder investor and boy its paying out for me.
And why not to pay your attention on some undervalued coins, which have high potential good technology and community support . Another great choice is DeepOnion - very good privacy coin, I think one of the best out there. There are a lot of new features is planned for 2018, so the price can easily make x2 or x3. Their dev team is getting larger. It's TOR network is one the strongest. They have largest number of TOR nodes so it is really untracable. They are developing new features like Deepvault online portal, Deepsend (similiar tech like Monero) and they are POS so anyone can profit while holding.
And, by the way, Onion still has running Airdrop campaign, which can boost your wallet by 5% weekly! Which, you must agree with me is always a nice bonus!
People are discovering it lately, Kucoin listed them and now Binance is annouced. Worth taking a look 4 sure.
Always DYOR I am not investment advisor, but it is clear shot from my standpoint of view.