the most important pairs are Bitcoin and Ethereum. These are two fundamental currencies, without which I see no sense in the exchange.
I agree these currencies are the base and always making some big movements by giving enough rooms for any trader to buy and sell but only with some accurate predictions. I mean to say without proper analysis you cannot trade both bitcoin and ethereum. You must need big knowledge and predictions capability. But, if you take some new coin, which is being backed by some good project then trading it will be very much easier as it will be having uptrend for sure.
The market is down, its good to trade on other alts, though having some trial and error on what to trade among new coins listed in different exchanges.
When the market is down, the truth is you cannot any pair with confident. Because, almost all the trading strategies are getting failed now a days hence staying away from the market is highly suggested as of now to secure our capital. When there could be no best pair for trading, it would be much better to stay away.