I don't think any of us can presume to know what the "best" cryptos to invest in are. Whether or not any digital currency could ever be fully anonymous is also questionable. But here is a list of some cryptos which are built around anonymity and have shown strong rates of adoption and subsequent price gains:
1. Digital cash (DASH) was formerly known as darkcoin. This is one of the most widely-held anonymity-based cryptocurrencies.
2. Monero (XMR) is an anonymity-based cryptocurrency which uses CryptoNote encryption technology. This crypto has seen decent price growth since its introduction.
3. Deep onion (ONION) is an anonymity-based crypto which uses the TOR network to make transactions difficult to trace. The TOR network is also used to power the anonymity-based TOR web browser. Although far less publicized than Dash and Monero, this crypto is has also seen relatively good adoption and price gain rates.
Hope this helps.