One that I can earn back the cost of the convention (cost of table, travel, lodging, printing). If I can break even, I consider it a success. Networking plus expenses earned back is huge for us, however, making a profit so we can do bigger and better (or even more smaller shows) is the true goal of a convention. If I don't break even then it's hard for me to justify going back to that show and losing more money on it. Networking is super important and sometimes worth the costs, however, I can only really afford one of those a year.
I've found that larger more popular shows are harder for independent artists to be successful at. The larger shows are more for nerd core fandom and cosplayers than for celebrating original work. Smaller more intimate shows, like comic book store events and small press expos, people go to those to browse for new reading material. I've also found that it's super easy to plan signing events with comic book stores, especially when you go to have fun with their costumers. Free sketch cards for kids, being open and chatting with them about art, the comic book industry, and how I've got my start, the stores I've been to love that and happily invite me back.
As an attendee at conventions, I go to scoop it out as a possible place for us to table at. I talk to the artist and ask them about how well they're doing at the show. If I hear a majority saying that it's a rough show, I cross it off my list. I'm always in business mode when it comes to comics, and I'm always looking for the most successful venues and method that I can make it my full time job.