Time of the day depends on the currency pair. Prefer trading at the times of their high liquidity. I trade EUR/USD when the London session and the New York session overlaps from 8:00 am to 11:00am ET.
For me this depends on where you live and your time availability. In the UK we are quite lucky to have the London and NY sessions smack in the middle for us. As such I like to trade EURUSD, GBPUSD mostly. I tend to use EURGBP as a gauge to see which of the two other trading pairs are likely to move better against USD.
I prefer trading EUR/USD even though the pair is a major currency pair it has enough volatility and also offers higher liquidity and tighter spreads.