If you have only one pair to pick, then pick EURUSD. This is undoubtedly, the most traded pair globally, they have light spreads, and a much more predictable movement.
Guidance provided by U.S federal reserves and ECB is outstanding (and this is one of the many reasons it is such a commonly traded pair indirectly), and the overall course of EUR and USD is linear and clear.
It is again strongly traded in North America and Europe during their sessions. It is good to focus in one or more currencies to gain a strong understanding of the trade and make money if a pair moves up or down.
USDJPY is also a very good currency pair to trade as a starter too. It has a predictable movement with low spreads since it is heavily traded in both Asian sessions and North American sessions.
It is very good to shorten the pair when you purchase yen as a safer haven especially during times of economic and political uncertainties. (USDJPY pair moves down while yen moves up.
(USD/CAD) is a cool deal also, even though its strongly traded in one session which is North America. it has a strong spreads and a predictable movement.
NZDUSD is very profitable too, but as it works predictably and moves gradually, others prefer it.