Can anyone explain when to use which pair and why?
you question is nearly impossible to answer, but I will try to get you to understand how this market moves, but first you need to understand this.
The whole market goes against USD and every Alt goes against both BTC.
In most cases we have the following
1- Bull market :
Case A: (Market Cap increasing , BTC dominance decreasing)
BTC/USD UP
Alt/BTC UP
Alt/USD UP+
(Best to trade USD pairs)
Case B: (Market Cap increasing , BTC dominance increasing)
BTC/USD UP
Alt/BTC Down
Alt/USD Nothing
(Best to trade BTC pairs)
Case C: (very rare)
BTC/USD Down
Alt/BTC UP
Alt/USD UP
(Something is wrong, don't trade go to bed)
2-Bear market :
Case A: (Market Cap decreasing , BTC dominance increasing)
BTC/USD Down
Alt/BTC Down
Alt/USD Down+
(Best to trade Alts/USD)
Case B: ( very rare)
BTC/USD Down
Alt/BTC UP
Alt/USD Nothing
(Best to trade Alts/BTC)
When markets are green, people take their USD , buy BTC and use some of that BTC to buy Alts, on the other side, when things are bad, people sell their Alts for BTC and sell their BTC for Dollars ( not always exactly the case, but this is a simple way of understanding the market)
I added the + sign to show you that when BTC pairs follow the direction of BTC/USD both gains and losses are higher on Alts/USD. so if you are trading ETH/USD and going long, and then a down move came, not only you are going to lose value to BTC but to USD too, so if BTC/USD goes down say 10% , then ETH/USD will go down say 15% because it also fell against BTC, and the whole market is measured against BTC.
Notice that by BEST , i am assuming you taking the right side of the trade, where profits are higher, if you go against the move, then your loses are greater, so the word best only stands for the best pair that is subject to have the largest move, whether in your favor or against it.
There is also side-ways market, where BTC/USD is almost flat, but Alt/BTC pairs do 20-50% moves a day, probably the best time to trade IMO.
But all that aside, it really depends on what you are trying to achieve, do you want more USD in your wallet? then trade only USD pairs, if you want more BTC in your wallet trade only BTC pairs.
That last point is really important, you can make a dozen of good trades on USD pairs, but at the end of the day you will have less BTC in your portfolio, you could trade tons of BTC pairs, end up with more BTC but much less USD in your portfolio.
I personally trade only BTC pairs 90% of the time, and that of course includes BTC/USD pair, if and when I get more
BTC in my account, it makes me feel good even if that means I lost some USD.