In my opinion, buy Bitcoin to get short-term profit because it is likely that the altcoins price will continue to fall to push Bitcoin price up. However, if you want a long-term profit, then choose the right time to buy some altcoins, specifically in the next 5-7 days. At that time, Bitcoin's price will level off and be a good time for Altcoins to follow.
Altcoins' prices are truly driven by Bitcoin's movement on the market. When Bitcoin is on the "green side", other altcoins follow. The same thing happens vice versa. In the short term, it's best to accumulate as much Bitcoin as you can before it's too late. There's the feeling that prices per BTC will become higher in the next months or 1-2 years from now because of the block reward halving. For the long term, you can try to buy and "hodl" prominent altcoins on the market such as Ethereum and Litecoin for big gains. The former will be transitioning from PoW to PoS, while the latter will include Mimblewimble privacy onto its own blockchain ledger. Things will become quite interesting for both of these cryptocurrencies over time, thanks to their active development and innovation.
When looking for good altcoins to buy, always take into consideration how actively developed an altcoin is and what use cases it brings to the mainstream world. There are hundreds of altcoins out there which are nearly identical to the original blockchains they've forked from, lacking mainstream adoption. Prices of a coin mean nothing, as they can be easily manipulated on the market (by whales, pumpers, etc.). What matters here is usability (and decentralization) above anything else. As long as you keep this in mind, you'll be successful in all of your altcoin investment ventures.
Nonetheless, I believe that the big winner in the altcoins' space will be Ethereum in the long run. The PoS upgrade among several other goodies, will make ETH attractive to institutional investors just like Bitcoin. I'd suggest anyone to buy and "hodl" as much ETH as possible before the upgrade takes place on July 2020 (hopefully). The excitement/hype will begin, leading towards a massive increase in price within the short term. In the long term, ETH might as well be worth $500 to $1,000 because of its undefined supply. Unless devs decide to make ETH a scarce cryptocurrency just like Bitcoin is today, it cannot be considered as a store of value. By investing in ETH and other top cryptocurrencies in market cap with active development and mainstream adoption, you can gain better profit opportunities on the market. Just my thoughts