It's Ethereum for me the king of all altcoins out there, it's the only altcoin that always try to show strength whenever bitcoin is pumping while others dump, eth follows bitcoin movements its just like a king (BTC) and a prince (ETH), litecoin also tries to follow bitcoin movements but not as much as eth, the only issue people find in eth this days is the high cost for gas fees, it's something that should be fixed before it gets out of hand, apart from that bitcoin and Ethereum are the right combination.