According to the DOT report released yesterday for the month of June, Southwest was most punctual, with 76.9% of flights on-time. Alaska, and Northwest were also significantly above average for the month. American had the worst on-time performance at 64.7%. The average was 70.9% for the 10 majors.
As far as flights that were regularily more than 70% late, American had the most, at 4.0% of all arrivals (77 flights averaged that amount late or more), and Alaska Airlines had the fewest (0). If you look at the past 12 years, Southwest has the best overall on-time record over that timespan. Some may say that they have some advantages in that department over other carriers, but you can't deny that they really try hard to get their flights on schedule.