I personally think the most successful ALT is BNB. It has been the best performing alt coin from the time of it's launch back in the July 2017 and since the beginning of the year BNB made a staggering 83% gain.
Although there are a lot of controversies around BNB coin, some call it a ponzi scheme and a centralized token. But hey,  isn't the whole crypto space a ponzi scheme for the people who lose money chasing the lambo and moon dreams and investing blindly without doing their own research.

**Not an investment advice**