Recently bought a little pile of beat, old (1960's) MAD Magazines from my pal, Sam Shiffer at Blue Moon Comics (Novato, CA).
Boy, If I had to pick one source that influenced me as a cartoonist it would easily be MAD Magazine. Just about every artist and writer in this wonderful publication was/is top notch and have influenced generations of artists and just just about everyone else... including me.
I prefer the issues that were first created in the 1950's up to, in-and-around the mid-1970's. I consider this first 20 year period MAD Magazines "prime years." Much beyond that... well.... nothing lasts forever.