I buy cereals from each one of those, but out of the three I buy the most kinds from Post.
General Mills has:
Cheerios
Chex
Fiber One
Lucky Charms
Total
Trix
Wheaties
Of those I only buy Cheerios (honey nut kind) and only from time to time.
Kellogs has:
Corn Flakes
Frosted Flakes
All-Bran Bran Buds
All-Bran Extra Fiber
All-Bran Original
Apple Jacks
Cinnamon Crunch Crispix
Cocoa Rice Krispies
Complete Oat Bran Flakes
Complete Wheat Bran Flakes
Corn Pops
Cracklin' Oat Bran
Crispix
Disney Buzz Blasts
Disney Hunny B's
Disney Mickey's Magix
Disney Mud & Bugs
Froot Loops
Honey Crunch Corn Flakes
Just Right Fruit & Nuts
Marshmallow Blasted Froot Loops
Mini-Wheats Honey Frosted Bite Size
Mini-Wheats Frosted Bite Size
Mini-Wheats Raisin
Mini-Wheats Strawberry
Mini-Wheats Frosted Original
Mueslix
Pokemon
Product 19
Raisin Bran Crunch
Of those I often buy all-bran and once in a while buy mini-wheats. Haven't even heard of some of the others and most are too sugary for my tastes.
Post has:
100% Bran
Alpha-Bits
Banana Nut Crunch
Cocoa Pebbles
Fruity Pebbles
Golden Crisp
Grape Nut O's
Grape Nuts
Grape Nuts Flakes
Great Grains
Honey Bunches of Oats
Honeycomb
Marshmallow Alpha-Bits
Oat Flakes
Post Toasties
Shredded Wheat
Shreddies
Sugar Coated Corn Flakes
Sugar Crisp
Waffle Crisp
Of those I often buy shredded wheat and shreddies and 100% bran..... so Post wins.
I add sliced fruit and/or fruit extract to my cereal to sweeten it up.