I can't say what I think is a "best" time, but I can comment on what it's been like to go on a weekday in August, having gone on two day trips during this time (one in 2003, one in 2004).
In 2003, we only went to
Disneyland Park. From opening time (9:00) until around 11:00, the park was pretty empty. We went from ride to ride in Discoveryland without waiting on line for any of them. After 11:00, the crowds thickened up a bit, and we had to queue up for several rides. The longest queue was 45 minutes, and that was for Thunder Mountain. Every other ride (except Peter Pan, for which we had Fast Passes) was under 30 minutes. All in all, though, we got to go on every ride in the park that day, eat at 2 sit-down restaurants, I did lots of shopping, we took a mid-day break just to sit and have a soda while people-watching, we saw the afternoon parade, we waited for a while to hold a good spot for and we saw the night parade, we saw the fireworks, and we took one last stroll on Main Street.
The only problem with the August 2003 trip? There was a huge heat wave in northern France at the time! Those of you from the southern parts of the UK might remember parts of it. It was so hot! I've never been in the Paris area on a hotter day before. Unfortunately, per French custom, most bottled drinks there are served warm and without ice. All of the non-French visitors were complaining quite a bit. But if you got a fountain soda or were in a sit-down restaurant is was not a problem.
We did another day trip in August 2004, this time starting at the Studios and then going to Disneyland Park. We were in the Studios by 8:45. We decided to make a lunch reservation for Blue Lagoon, as we had a great experience there the year before. We made the reservation for 13:30, figuring that if we missed anything in the Studios, we would come back.
Once the Studios opened, we basically ran from attraction to attraction, pure commando-style. There were basically a good number of people there, but no crowds at all. Like all of the shows were about 95% full (including the Car Stunt SHow, which seats many), but we never had to wait in line more than 5 minutes. As a result, we had seen almost everything (the exception being the sound-effects exhibit) by 12:00. We would have preferred to take it easy a bit, and not run from Animagique to Rock-N-RollerCoaster to the Stunt Show, but since we only had one day to visit, this was fine. We left the Studios around 12:30 (did some shopping before leaving) and headed to Disneyland Park. We considered going back to the Studios for the parade, but it seemed like it would have been a hassle. At Disneyland Park, the crowds were not bad, but noticeable. Again, everything except Peter Pan and Thunder Mountain had a queue of less than 30 minutes. We did not see everything on this trip, and just picked our favorites, including both parades, 2 sit-down meals, the fireworks, and shopping for me.
The biggest difference from 2003 to 2004? Temperature!!! After sweating all day in 2003, it actually got a bit nippy at times in 2004. We were quite glad to have our sweatshirts during the day, and I actually was fairly cold at night. Instead of ordering a cold San Pellegrino at dinner, I needed a hot cup of tea!
