Any alt coin suggestion expect ada.In my point of view of ada still can't surpass ETH in the smart contract field and even if they can make it will be hard to be replace existing eth smart contract eco system.Moreover late coming project like sol even surpassing to ada,all things ada have is the loyal user base and nothing special things in tech
Bitcoin is the first choice when you talk about best and most secure coin for long term investment for long term. As far as Alts are concerned there are many good projects which have huge potential of growth after Bitcoin are as follows: ADA, POLKADOT, LUNA, SOLANA and MATIC , all of them have huge potential to grow in coming weeks and months becuase their use case is strong as their platform is used to create DEFI smart contracts as well for creating NFTs which are in huge demand nowadays.