The BEST one was VACATION and after that they went down hill (And continue to)
The first VACATION was great, but I also like
Christmas Vacation. The other two are hit or miss. They have some good moments, but overall, don't hold up for 90 minutes.
But IMO the best Lampoon movie is the first,
Animal House. Here you had the cream of the
Lampoon writing staff (including the legendary founder of the mag, Doug Kenney--who appears in the film as Stork) crafting a rude, crude classic satire. The original script was even more outrageous than what ended up on screen.