Right now it is probably monero. It does one thing (anonymous transactions) and does it alright.

But I think we will see increased adoption of less decentralized and anonymous systems like GNU Taler or Nano for micropayments, which are generally lower-risk and hinge more on per-transaction fees. I think that is the "real" use-case of cryptocurrency besides buying drugs on the DN- to serve as an micropayment/spam-prevention system and an alternative to advertisement-based funding for websites.

Output obfuscation is a really useful feature, but it comes at a pretty great cost of bandwidth. I think this area will continue to improve in the future.