For now, Ethereum is the best altcoin, no other altcoins are better than Ethreum, including Cardano. However, it is not impossible if someday Cardano can pass Ethereum, but it is likely not to happen in the near future. Ethereum seems to get stronger day after day, even BNB cannot pass Ethereum so far. People already trust Ethereum as the king of altcoins and be the most favorite altcoin for years. So, it is very difficult to replace it as the best one.