I think the best time is when there is volatility in the market and worse time is when there is not traders in the market.
There is four trading sessions during day, and each one have different volatility on each pair.
But generally the best time is when Europe and US market is open. You can see here more details about trading sessions.
When Europe and US market is open then there is many traders who trades majors and thus volatility comes to the market.
For you it is the best when you have liquidity in the market where you can open and close trade whenever you want. It should not happen that you open order and you get re-quote because there is no one willing to accept your order.