Pronto Pup. There's the one in Rockaway Beach, there is another in Seaside next to the bumper cars. I understand it isn't IN Portland, but it's the best damn Corn dog around. If you realllly one a Corn Dog in city limits, check out the beast that Loyal Legion is making with Olympic Provisions.
You can get all the fixin's to make a pronto pup in your house on their website. Best of all, Pronto Pup is a local company. Get ya' some.