Well, for Best:
90% of cereal mascots... my personal favorites- Cap'n Crunch, Quisp, Quake, Tony the Tiger, Toucan Sam, Fred Flintstone (yeah, he counts), Barney Rubble, Count Chocula, Sonny the Cuckoo.
The M&M's. My personal favorites were Yellow Peanut and Orange Crispy (who lasted longer than Crispy M&M's BTW). I have all the Star Wars PVC's of them.
Grimace and Hamburgalar. Clearly the best of the McD's characters.
the Drake's cakes duck (I always thought he was cute).
The Enegizer Bunny.
The Wendy's "Where's the Beef" Lady.
Now for the Worst:
Burger King King puppet. If there's anything worse than the BK King, it's the little plastic puppet in the BK Kids meal ads. How patronizing can ya get? I also hate the "Whopper" and "Whopper Jr." guys too.
The Milk Milk carton, with a guy's head clearly popping out of it. In a cheap attempt to cash in and steal the similar commercials with Crash Bandicoot, going up to random people on the street (staged) and bugging them for attention wasn't funny the first time. Too bad every commercial is like that now, but good news is that these ads have stopped years ago.
The poorly drawn stick figure dog from the "Anti-Drug" commercials. What the heck was that supposed to be?
And on the subject of dogs, the Cottenelle puppy. I just want that dog to SHUT THE HECK UP! It's not cute. it's obnoxious.
The Honey Comb Fuzzy thing. And the Big Mouth kids. Bad, bad, bad.