Almost all top coins have sunk heavily in price. Also, tokens of young projects can be purchased at a price lower than what they cost on ICO. Top coins fell in price less than others, but investing in them is less risky. You can collect a conservative investment portfolio of top coins. For example, Bitcoin, XRP, Ethereum, Cardano, Stellar, EOS.
Bitcoin isn't altcoin, your suggestion only buy top altcoin on cmc and that's it. What OP want is he wants buy altcoin which still on low, but has potential to rise up in the future