The following are my candidates. If you'd like to make a case for another one, please do so in the comments.
Obviously, you can't make a list like this without mentioning Charlie Brown's hygienically-challenged pal, Pigpen. Anyone who is forever enveloped in a cloud of fecal dust has got to be more than a little ripe. Shit, just look at him. I seriously doubt this fucker wipes.
2. Broom Hilda
I see Broom Hilda and the first thing that pops into my mind is the smell of a cauldron full of moldy onion juice. It's been rumored that Broom actually supplements her witch salary by selling the abundance of radishes that grow between her toes at a local farmer's market.
3. Hagar The Horrible
If my family heritage has taught me anything it's that Scandinavians should never be underestimated when it comes to their ability to "bring the funk". I'd wager you could wring about a quart of beer farts out of that burlap tunic of his.
While contemplating this post, I asked Megan which character she thought smelled the worst. She thought Hagar probably smelled bad and then added, almost apologetically, "I bet, um...Cathy probably smells, too." It stands to reason. She seems to have everything else going against her. Why not chronic body odor? Plus, like Pigpen, she strikes me as a non-wiper.
5. Funky Winkerbean
This one is my dark horse, but one who deserves consideration. Sure, he looks sanitary, but I highly doubt he got the name "Funky" because of his love of Bootsy Collins.