Last friday (6/25/2010, Friday once again), passengers in Shanghai Metro reached a new high. It is up to 6.11 millions in total of daily Metro commuters, Shanghai Expo visitors, and weekend shoppers. Compared to the previous record, 100k more passengers have packed into metro, and more than double visitors chose line 13 to enter expo park now.
Currently, Shanghai Metro has 11 lines under operation, and Line 13 is an extra line dedicated to Shanghai Expo 2010 only. However, most packed lines are still Line 1 and Line 2 which are the earliest Metro lines in Shanghai. Passengers who took Metro line 1 and Line 2 on last Friday reached 1.25 and 1.17 million respectively.
So try to take Metro at off-peak time or Line with less traffic as you could. (more metro tips here)
Which line is most packed in a day, read diagram below. X axis is daily passenger number, and Y is Metro Line number.

Shanghai Subway Daily Passengers