this question invariably pops up once every month or two. As has been said on others (and as moon quoted corty as saying above) there is no "best time". If there were, then it wouldn't be as we would ALL pile on there, making it the worst time for visibility purposes.
So what's the best time? The same time. If you publish at 5pm/1700 your time, then posting at 5pm/1700 consistently is the best option. It might be considered dirty, but if you have unrelated chapters (like meta chapters, posting an apology for an absence, posting an additional chapter for a poll, etc) then use THAT chapter for a different day/time to be seen by new readers. Otherwise, expect that it's a slow thing to grow and be seen. I've been doing this on this site for coming up on two years and I'm still growing ever so slowly.