There is no best altcoin to invest. Try to diversify - have some high price coins, like ETH or NEO. You can have some on the middle level too. You can also try to find you own gem which could give you x10, x20... Just remember that there are no simple solutions, like there is one best altcoin to invest.
I think best altcoin to invest in presently are ethereum, NEO, litecoin, waves and EOS because they have very good potentials but you can also look out for new token that also have very good potentials because you never can tell how big they will be in future.