for me for the long term I just choose bitcoin and that is enough, unless we want a bigger profit, then we can choose another altcoin that is at the top of the coinmarketcap, where these altcoints have a greater chance than other altcoins , but if we understand the market movement, we can wait for the best price to buy
BTC should be the primary coin for investment in crypto. Its also good to build a diversified portfolio with BTC having majority share while remaining share be given to top alt coin of CMC like ETH, LTC, Monero etc.
Its very unlikely that alt coins may go up without btc price going up. That's one parameter we have to keep in mind while investing in alt coins.