If you want the best advice mate, then it is DYOR, that is the first and most important thing to do before any investment whatsoever, after doing your own research, you might not need anybody's advice on what to do or the right steps to take; you should also take into cognizance that most altcoins are short term projects and hodling them for quite a long time might not be the best thing to do.
Having said that, Ethereum has been the best altcoin for quite a long time now, with many Eth killers coming and failing, i have to select Ethereum as one of the best altcoins an investor can invest in; and if you are to choose Ethereum, then you don't have to buy any other coins because you think you want to diversify, you are only trying so hard to lose money, if you choose one coin, and you want to diversify, then you look outside of the crypto market for another investment.