My ratings (worst to best):

Roy Gethard: 6/10

Peach: 4.5/10

Muriel: 3.5/10

Puppet Zack: 1/10

Nigel: 0.5/10

Dewey Donedidit; 0.5/10

I only count characters that have more than one appearance, so Peach is the worst

Nigel

I really don't understand the hate behind Dewey.