It all depends on your ability to take risks. The safest investment for a long term is bitcoin or ethereum, of course. Altcoins are more risky, but you can expect a big profit by investing in this part of the market.
It depends on how long to invest money. If for a year or two, then I would agree. If for a longer period, then I'm not sure that Bitcoin and Ethereum will be still on top. Many competitors have come to this market.