Breck has lots of great terrain, but what of it will be great depends on the conditions at the time. One way to maximize your time would be to take either a group lesson ($110) or the big mountain experience ($190). You'll then get a top pro to show you the mountain -- something that is hard to get on your own.
As for the classic lines, here's my take.
1. The steepest terrain at Breck, AFAIK, is in the Lake Chutes. You'll need to hike 130 vertical feet above the top of the Imperial Express, topping out at 13,000 vertical feet. The steep lines are Crazy Ivan's and the much tougher Elevator Shaft (which I've seen 1 snowboarder ride -- it is pretty close to a 40 foot vertical cornice), and Zoot Chute is pretty nice but still steep. A mellower line is to continue past the Lake Chutes to Snow White for some tasty powder if it is a powder day.
2. I generally think of the Imperial Express as a lift to get somewhere else -- I don't find Imperial Bowl to be very interesting. That being said, it is also the way to access Peak 7. Sidestep up 10 feet and drop into the Whale's Tail and drop the cornice. Make sure to traverse skier's left onto Peak 7 (you'll encounter a number of traverses), then ski down the ridge until you come close to the nose -- you can drop over the edge and come back to the ridge kind of garlanding your way down for fun, if you like. At the nose, you've got a couple of choices. Straight off along the boundary is Art's Bowle, and it is really lovely when their is a bunch of new snow on it. Skier's right is Magic Carpet, a 45 degree shot to the floor.
3. Horseshoe Bowl is something not to miss. You can reach it by the T-Bar, or by taking the high traverse from the top of the 6 chair and scooting around to the Bowl. The t-bar line is better, but the wind is sometimes fierce on the t-bar. Move left after encountering the choke point and you've got probably 700 vertical feet of 45 degree plus slope -- and a big canvas on which to paint your turns.
4. Using the high traverse noted above, you can ski down the ridge line and drop into Contest Bowl, a particularly steep but short shot. Very tasty.
5. There's lots of interesting lines on E Chair. For a tight steep shot, ski MineCrotch, which is a tree alley between Mineshaft and Devil's Crotch. It can be a bit challenging getting into the area with your bases intact, but what are skis for? You take the traverse towards Mineshaft, but about half way down, veer left down a 10-12 foot wide set of tree shots in pretty steep terrain.
Those are a few ideas, but you really won't get a view of the area without a guide. Hence, either find a local to guide you, or think of the options outlined above. Snowpro, aka Jonathan Lawson, sometimes is on this board and is an excellent instructor at Breck. You might try to get him if you do either the lesson or BME -- I can assure you you will have a great time.