I guess Frosted Flakes. Rice Krispies were pretty good if I could sneak extra sugar onto them.
Of the cereals Mom would allow us to have - whichever had the best toys, or things you could send in a few box tops to get (so no Chocolate Frosted Sugar Bombs, no matter what toys they had).
Top of the list were 3-D baseball and football cards. I'd even eat Wheaties and Rasin bran to get those.
I once sent in some box tops and a few bucks for a 4' long blow-up shark from Finding Nemo. Blew it up and left it in the living room. Came home, it was completely deflated and had about 1000 cat claw holes in it. I wish I had that video.
Favorite: Lucky Charms and original Shredded Wheat.