Bitcoin and ethereum have both been impressive, but in my mind the outstanding performer is litecoin, currently the 5th largest by market cap. Large enough to avoid all of the junk ICOs and scams, but small enough to have flown under most people's radar. Given it's similarities to bitcoin, and the fact that it's supply is roughly four times as large as bitcoin (84 million litecoins versus 21 million bitcoins), it has the potential the play "silver" to bitcoin's gold and catch up the "silver/gold" ratio. It will probably never hit the natural ratio of 4:1, but I think it is very likely when litecoin is discovered it will achieve a 10:1 ratio relative to bitcion, which would be about $400 now to bitcoin's $4000, implying a 5X increase from litecoin's current $80 valuation, not even figuring in further long-term price gains in bitcoin.