My comics are always the #1 seller, no matter the con! If the comic isn't doing well usually I'm not doing well. =v=; and I bring fanart prints and buttons with me, too.
Even considering my highest turnout with prints, I sell 3 times the amount of comics. (At my highest for a weekend, total prints sold were 55, and total comic issues was 168, just to illustrate the divide)
Prints are going to have the highest profit margin because people are paying for the art, not the printing. Your next most profitable is going to be trade paperbacks, followed by things like charms/keychains, comic issues, bookmarks, badges, and buttons. That isn't to say you shouldn't stock those lower ticket items, you just have to sell more of them to make up your costs. Some people make bank on buttons alone!
Just remember that when selling at a con, low quantities of a large variety of items is key. You want to be able to draw in a lot of different people. Start with only 10-15 of each item (prints and other merch), unless you're SURE it'll sell. Maybe 50-100 of a comic if you're just starting out and only plan to do a couple cons.