I don't think there's a single "best" time that guarantees you'll win, but the time you play can definitely change the type of opponents you face.
In my experience, evenings and weekends tend to have more casual players online since that's when most people are off work or school. During quieter weekday hours, you'll often run into more regular grinders who play consistently.
That said, good bankroll management and making solid decisions are far more important than trying to find the perfect time to play. If you're consistently making better decisions than your opponents, you'll do well regardless of the time of day.
I'm curious what everyone else has noticed. Have you found certain times or days where the games seem noticeably softer?