Bitcoin should be the main choice but remember that is not so volatile as another coin and that's why people must try to divide their money and they must also invest to the altcoin. At least promising altcoin will give same guarantee with what already given by bitcoin too. It's not a big risk by investing your money in the top altcoins. Everyone understand properly about that how major altcoins are giving guarantee.
Bitcoin may be the biggest one but altcoin may give you more profit than bitcoin. Look at some coins like DASH, BCH and many more. they are getting very huge increase at this moment.