Regarding the question, what currency is best for trading for ex, I'll summaries it in the best way I understand it.
Though I particularly trade with gbpusd because i took my time to study the moevent pattern, both in the morning and in the evening.
Those movements have been affected lately due to Brexit.
But all in all, try to choose any pair of your choice, preferably those that attracts little spread and study them for a long time so you can know their behavioural pattern and movement.
This helps to reduce risk seriously instead of jumping from one pair to another.