That is a simple question with a complicated answer, which depends on a lot of things - what kind of spreads do you prefer, are you a long-term or a short-term trader, which pairings are you interested in to begin with, etc.etc.
@mlawson71 is definitely correct. there's a lot to consider. however, to answer your question, the most common traded pairs are EUR/USD, GBP/USD and USD/JPY. But I will provide a link that contains brief articles to increase your knowledge about different currency pairs.
icepower72 posted: here is another interesting question - is there a pair that you could call 'the most predictable' ..., anyone?
That depends on the trader, some who specialise in one currency have a higher hit rate, even when it doesn't behave as predictable they know it will revert, for me I would say eur/gbp but its a personal thing.
I think predicting any currency pair is just a probability of either it will go up or down or will not. Everything depends on market fluctuations, sometimes geopolitical issues and disasters are responsible for a rate change in Forex Pairs which can't be predictable so as per my experience, there is nothing like the most predictable.
To me a predictable pair is the one that often moves smoothly within its trend without much choppiness. But different pairs fit that description at different times, so one should always be paying attention.