Darkcoin is by far the best developed alt-coin ever. With its RC5 Launch (today) and upcoming INSTANT confirmation no one could touch them.

(should you wish to know more: "Transaction Locking and Masternode Consensus: A Mechanism for Mitigating Double Spending Attacks Document version: 1, published September 19th, 2014" @ https://www.darkcoin.io/downloads/InstantTX.pdf tells you all you need to know)