Stan isn't dumb at all, dude is a CIA agent...he is just overzealous...Peter is that dumb more so because he chooses to be an ignorant jerk because he is lazy, but Homer is completely clueless and actually shows he regrets being so clearly stupid.
I say Peter Griffin, Homer Simpson has a crayon stuck in his brain and Stan Smith works for the government so both have a reason to be dumb while Peter is just naturally dumb.