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
The best altcoins for long term for me was Matic(Polygon), Xvg(verge), Shiba Inu(Shib), Solana(Sol)
and BSW(Biswap), but of course there are other altcoins out there that are great and good aside from this
these are just I think best for me only.