Holy crap. This sucker could be way long. I've been doing this for thirteen 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 and producer for production companies or on my own. 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 Esty
Campbell's Soup
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
Grey Advertising
GSD&M
Harrah’s
Intel
J. Walter Thompson (Atlanta, Detroit, San Francisco)
Jerry Springer
K-Mart
Ketchum Advertising
La Choy
Leo Burnett
MGM
Marshall Field's
McCann-Erickson (Atlanta, Houston, New York)
Novartis
Ogilvy & Mather (Chicago, LA)
One And All
Pepsi
Perkins
Regis
The Richards Group
Rite Aid
Seven Up
Slumberland
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