Depends where you live, what time of day is good. Some of the best times can be when you get people coming home drunk - if you live in the UK, that can mean finding good games first thing in the morning as you're playing against US players who are drunk. Drunk players can play very aggressively, though, so they can be harder than loose/passive players.
Lunchtime players can also be good - since they're only able to play for a limited amount of time, they want to get some action, so they'll play more
hands than they would otherwise.