Holy crap. This sucker could be way long. I've been doing this for twenty years! So from memory, some unfiled files and a pretty updated data base, I chose direct clients, products or agencies to represent each letter of the alphabet. I've either worked with them as a writer or producer or both. I chose those I thought you might like and definitely those I like. (As agency names tend to change, I've attempted to list them as they were when I last worked with them.) Here goes:

Ammirati Puris Lintas (New York)
Arby's/Sbarro
AT & T
Baby Ruth
BBD&O (Atlanta, New York, Toronto)
Best Buy
Campbell Mithun
Campbell's Soup
Carmichael Lynch
Clarity Coverdale Fury
DDB (Chicago, Dallas)
Direct TV
Disney Channel
DMB&B (St Louis, Troy)
Eisner & Associates
Euro RSCG & Partners (Salt Lake City)
Fallon
Federal Express
General Mills
Gerten’s
Grey Advertising
GSD&M
Harrah’s
Intel
J. Walter Thompson (Atlanta, Detroit, San Francisco)
Jerry Springer
K-Mart
Ketchum Advertising
La Choy
Lampert Exteriors
Leo Burnett
MGM
Marshall Field's
McCann-Erickson (Atlanta, Houston, New York)
Novartis Ogilvy & Mather (Chicago, LA)
Pepsi
Perkins
Regis
The Richards Group
Rite Aid
Seven Up
Slumberland
Surdyk’s
TBWA Chiat Day (San Francisco)
Tostitos
US West
Valley Automotive Group
Visa
W.B. Doner (Baltimore)
Wendy's
Young & Rubicam (Chicago, New York)
Zullo & Associates