Cannon can definitely deliver on the steep stuff. I don't know what their steepest trail is.
Tramline definitely has the steepest section; you could probably ski it without airtime if you picked your route really carefully. It's rarely open, although it is open now. Bring your rock skis if you want to try it.
Paulie's, Zoomer and Avalanche are pretty steep for a couple hundred vertical. Zoomer usually has a totally-scraped-off groomer path down the middle of it. Paulie's is never groomed. Avalanche -- sometimes it'll be groomed, and sometimes it'll be bumped. Avalanche just might be the steepest trail at Cannon, at least off the Paulie's Extension entrance.
Middle Hardscrabble is probably the hardest trail at Cannon that's open with any regularity. It's never groomed, it's narrow(ish), it has more turns then a slinky, and it's steep. If you can non-stop MH --
There are a couple other blacks (Profile, Skylight and Extension) that aren't quite as fearsome as those. If you're still looking for challenge -- head on over to Mittersill. Out-of-bounds. Lots of people do it; ask somebody going over if you can hang with them.