This was #57, the December 1967 issue of Sick.
Avis Rent-a-Car had a slogan in the sixties about how they were the second best rental car company which was parodied by many other products. That's what the "Why try harder?" button's about. Sick was under the impression they were the second most popular humor magazine, which would be true if issues of the magazine were as easy to find as Cracked or Crazy.
Not sure who the cover artist is. Joe Simon maybe?
Harry “The Professor” Borgman