Buy-and-hold crypto trading has definitely been one of the most profitable cryptocurrency trading strategies so far. This is not surprising because most of the major cryptocurrencies are not far from their all-time highs and many are pushing to fresh highs at the moment. The guys who have bought and held these cryptos during the last few years until now have all been rewarded handsomely.
Always pay attention to Bitcoin. Most altcoins (every cryptocurrency except Bitcoin) are pegged more closely to Bitcoin than Asian currencies were to the USD during the Asian Financial Crisis. If Bitcoin price pump drastically, altcoins price can go down as people try to exit altcoins to ride the BTC profits; inversely, if Bitcoin prices dump drastically, altcoin prices can go down, too, as people exit altcoins to exchange back into fiat. The best times for altcoin growth appear when Bitcoin shows organic growth or decline, or remains stagnant in price.