I have to say Ark.io Ecosystem, it's the number one altcoin in my book technology wise.
Having a robust technology doesn't guarantee success in the market. If that was the case, then third generation cryptocurrencies such as Cardano and NEO could have overtaken Bitcoin many years ago. But that hasn't happened and these coins are now outside the top-10 list as well. Along with technology, there are other factors that needs to be considered, such as the competence of the development team, acceptance of the coin and daily trading volumes.