The first show I ever saw was Snow on the Informer tour in 1994ish. He wasn't even the worst though, he had an opener who went by Rumble, who had a hit on MuchMusic in the early 90's that has been forgotten to time, but he was the worst. I don't think he even performed any songs, he just went up there, cursed a lot then left the stage. Everyone was like WTF?
As far as bands that maybe people have heard of, I Mother Earth (another 90's Canadian act), at Edgefest '96 was pretty unbearable. Like zero talent between all four "musicians". Crazy how many bands made it big back then based solely on their looks due to the whole music video revolution.
More recently, and Canada GAF is going to HATE me for this one, the Tragically Hip in 2007 at 10,000 Lakes Festival in Minnesota was probably the worst show I've seen that I can still remember vividly. They were given an amazing late night spot on the big stage right after the headliner (String Cheese Incident) finished their set. I wasn't a big Hip fan going into it, but the amount of disdain, or animosity they showed towards that crowd was embarrassing. I went to their show, as opposed to the other show because I'm Canadian and wanted to show some support for one of the few Canadian bands at this festival, but after this set, I don't think I told anyone I was Canadian the whole weekend.
This is like right after Hurricane Katrina and Gord goes up on stage, pissed drunk (which I'm fine with, it's a late night set), and before New Orleans Is Sinking, just yells "New Orleans is sinking and I don't fucking care!!!", and I don't know if he was expecting people to cheer or what, but it was just dead silence. And then it was some of the sloppiest playing and singing I'd ever heard from any band. Like, if they were playing a dive bar and there were three people in the crowd, they would probably be asked to leave the stage.
There were two bands playing at the same time, and the Hip were definitely the bigger of the two bands playing, on the bigger of the two stages, but by 3 or 4 songs into their set almost everyone went to see the smaller band (Trampled by Turtles) at the smaller stage, or just went back to camp since there wasn't a ton of room at the smaller stage.