I think it depends on the season. In the last three weeks, I went on Friday twice, Saturday twice and Sunday twice. The first Friday and Sunday was PACKED!! Saturday was really nice, not a big crowd. But the second time around, it was the opposite, Friday and Sunday were cool but Saturday was PACKED!! I would think now that school is back, I would say Saturday is the busiest because a) Fridays would be half day for most, so maybe it wouldn't be worth it for those without passes. Sunday, would most likely be travel days so travelers wouldn't be willing to stay in a park all day and travel all night.
Least crowded that's easy it's Saturday. I went 09/05/09 and most wait times were less than 15 minutes or walk on. If you get to the park at opening and leave at midnight you probaby go on every ride. In the mornings Fantasyland is very empty. Also fastpasses help to save time. This might change due to Space Mountain and Haunted Mansion being closed. It's true Sunday at night Disneyland has no crowds but the park closes at 11 at night.