Cardano (ADA) has the most potential to overtake Ethereum. There are more and more ICOs launched on NEO but they are more focused on China so it's not a direct competitor.
Zilliqa just started trading and is already 1300% above ICO price. It's still below $1 (currently $0.132), it has potential to do another 500% in the next few days!
It for sure started well. Hope it's gonna continue this way for a bit longer, looking forward to 3rd generation blockchains like Cardano and Aion start getting some real value.